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  <div class="section" id="module-dataclasses">
<span id="dataclasses-data-classes"></span><h1>30.6. <a class="reference internal" href="#module-dataclasses" title="dataclasses: Generate special methods on user-defined classes."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dataclasses</span></code></a> — Data Classes<a class="headerlink" href="#module-dataclasses" title="本標題的永久連結">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/dataclasses.py">Lib/dataclasses.py</a></p>
<hr class="docutils" />
<p>This module provides a decorator and functions for automatically
adding generated <a class="reference internal" href="../glossary.html#term-special-method"><span class="xref std std-term">special method</span></a>s such as <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> and
<a class="reference internal" href="../reference/datamodel.html#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> to user-defined classes.  It was originally described
in <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0557"><strong>PEP 557</strong></a>.</p>
<p>The member variables to use in these generated methods are defined
using <span class="target" id="index-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0526"><strong>PEP 526</strong></a> type annotations.  For example this code:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">InventoryItem</span><span class="p">:</span>
    <span class="sd">&#39;&#39;&#39;Class for keeping track of an item in inventory.&#39;&#39;&#39;</span>
    <span class="n">name</span><span class="p">:</span> <span class="nb">str</span>
    <span class="n">unit_price</span><span class="p">:</span> <span class="nb">float</span>
    <span class="n">quantity_on_hand</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span>
    <span class="k">def</span> <span class="nf">total_cost</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">float</span><span class="p">:</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">unit_price</span> <span class="o">*</span> <span class="bp">self</span><span class="o">.</span><span class="n">quantity_on_hand</span>
</pre></div>
<p>Will add, among other things, a <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> that looks like:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">unit_price</span><span class="p">:</span> <span class="nb">float</span><span class="p">,</span> <span class="n">quantity_on_hand</span><span class="p">:</span> <span class="nb">int</span><span class="o">=</span><span class="mi">0</span><span class="p">):</span>
    <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
    <span class="bp">self</span><span class="o">.</span><span class="n">unit_price</span> <span class="o">=</span> <span class="n">unit_price</span>
    <span class="bp">self</span><span class="o">.</span><span class="n">quantity_on_hand</span> <span class="o">=</span> <span class="n">quantity_on_hand</span>
</pre></div>
<p>Note that this method is automatically added to the class: it is not
directly specified in the <code class="docutils literal notranslate"><span class="pre">InventoryItem</span></code> definition shown above.</p>
<div class="versionadded">
<p><span class="versionmodified">3.7 版新加入.</span></p>
<div class="section" id="module-level-decorators-classes-and-functions">
<h2>30.6.1. Module-level decorators, classes, and functions<a class="headerlink" href="#module-level-decorators-classes-and-functions" title="本標題的永久連結">¶</a></h2>
<dl class="function">
<dt id="dataclasses.dataclass">
<code class="descclassname">&#64;</code><code class="descclassname">dataclasses.</code><code class="descname">dataclass</code><span class="sig-paren">(</span><em>*</em>, <em>init=True</em>, <em>repr=True</em>, <em>eq=True</em>, <em>order=False</em>, <em>unsafe_hash=False</em>, <em>frozen=False</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.dataclass" title="本定義的永久連結">¶</a></dt>
<dd><p>This function is a <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a> that is used to add generated
<a class="reference internal" href="../glossary.html#term-special-method"><span class="xref std std-term">special method</span></a>s to classes, as described below.</p>
<p>The <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> decorator examines the class to find
<code class="docutils literal notranslate"><span class="pre">field</span></code>s.  A <code class="docutils literal notranslate"><span class="pre">field</span></code> is defined as class variable that has a
type annotation.  With two exceptions described below, nothing in
<a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> examines the type specified in the variable
annotation.</p>
<p>The order of the fields in all of the generated methods is the
order in which they appear in the class definition.</p>
<p>The <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> decorator will add various 「dunder」 methods to
the class, described below.  If any of the added methods already
exist on the class, the behavior depends on the parameter, as documented
below. The decorator returns the same class that is called on; no new
class is created.</p>
<p>If <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> is used just as a simple decorator with no parameters,
it acts as if it has the default values documented in this
signature.  That is, these three uses of <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> are
equivalent:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="o">...</span>
<span class="nd">@dataclass</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="o">...</span>
<span class="nd">@dataclass</span><span class="p">(</span><span class="n">init</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="nb">repr</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">eq</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">order</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">unsafe_hash</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">frozen</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
   <span class="o">...</span>
</pre></div>
<p>The parameters to <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> are:</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">init</span></code>: If true (the default), a <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method will be
generated.</p>
<p>If the class already defines <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a>, this parameter is
ignored.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">repr</span></code>: If true (the default), a <a class="reference internal" href="../reference/datamodel.html#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> method will be
generated.  The generated repr string will have the class name and
the name and repr of each field, in the order they are defined in
the class.  Fields that are marked as being excluded from the repr
are not included.  For example:
<code class="docutils literal notranslate"><span class="pre">InventoryItem(name='widget',</span> <span class="pre">unit_price=3.0,</span> <span class="pre">quantity_on_hand=10)</span></code>.</p>
<p>If the class already defines <a class="reference internal" href="../reference/datamodel.html#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a>, this parameter is
ignored.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">eq</span></code>: If true (the default), an <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> method will be
generated.  This method compares the class as if it were a tuple
of its fields, in order.  Both instances in the comparison must
be of the identical type.</p>
<p>If the class already defines <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a>, this parameter is
ignored.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">order</span></code>: If true (the default is <code class="docutils literal notranslate"><span class="pre">False</span></code>), <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a>,
<a class="reference internal" href="../reference/datamodel.html#object.__le__" title="object.__le__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__le__()</span></code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__gt__()</span></code></a>, and <a class="reference internal" href="../reference/datamodel.html#object.__ge__" title="object.__ge__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ge__()</span></code></a> methods will be
generated.  These compare the class as if it were a tuple of its
fields, in order.  Both instances in the comparison must be of the
identical type.  If <code class="docutils literal notranslate"><span class="pre">order</span></code> is true and <code class="docutils literal notranslate"><span class="pre">eq</span></code> is false, a
<a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised.</p>
<p>If the class already defines any of <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a>,
<a class="reference internal" href="../reference/datamodel.html#object.__le__" title="object.__le__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__le__()</span></code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__gt__()</span></code></a>, or <a class="reference internal" href="../reference/datamodel.html#object.__ge__" title="object.__ge__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ge__()</span></code></a>, then
<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> is raised.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">unsafe_hash</span></code>: If <code class="docutils literal notranslate"><span class="pre">False</span></code> (the default), a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method
is generated according to how <code class="docutils literal notranslate"><span class="pre">eq</span></code> and <code class="docutils literal notranslate"><span class="pre">frozen</span></code> are set.</p>
<p><a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> is used by built-in <a class="reference internal" href="functions.html#hash" title="hash"><code class="xref py py-meth docutils literal notranslate"><span class="pre">hash()</span></code></a>, and when objects are
added to hashed collections such as dictionaries and sets.  Having a
<a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> implies that instances of the class are immutable.
Mutability is a complicated property that depends on the programmer’s
intent, the existence and behavior of <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a>, and the values of
the <code class="docutils literal notranslate"><span class="pre">eq</span></code> and <code class="docutils literal notranslate"><span class="pre">frozen</span></code> flags in the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> decorator.</p>
<p>By default, <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> will not implicitly add a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>
method unless it is safe to do so.  Neither will it add or change an
existing explicitly defined <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method.  Setting the class
attribute <code class="docutils literal notranslate"><span class="pre">__hash__</span> <span class="pre">=</span> <span class="pre">None</span></code> has a specific meaning to Python, as
described in the <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> documentation.</p>
<p>If <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> is not explicit defined, or if it is set to <code class="docutils literal notranslate"><span class="pre">None</span></code>,
then <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> <em>may</em> add an implicit <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method.
Although not recommended, you can force <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> to create a
<a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method with <code class="docutils literal notranslate"><span class="pre">unsafe_hash=True</span></code>. This might be the case
if your class is logically immutable but can nonetheless be mutated.
This is a specialized use case and should be considered carefully.</p>
<p>Here are the rules governing implicit creation of a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>
method.  Note that you cannot both have an explicit <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>
method in your dataclass and set <code class="docutils literal notranslate"><span class="pre">unsafe_hash=True</span></code>; this will result
in a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p>
<p>If <code class="docutils literal notranslate"><span class="pre">eq</span></code> and <code class="docutils literal notranslate"><span class="pre">frozen</span></code> are both true, by default <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> will
generate a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method for you.  If <code class="docutils literal notranslate"><span class="pre">eq</span></code> is true and
<code class="docutils literal notranslate"><span class="pre">frozen</span></code> is false, <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> will be set to <code class="docutils literal notranslate"><span class="pre">None</span></code>, marking it
unhashable (which it is, since it is mutable).  If <code class="docutils literal notranslate"><span class="pre">eq</span></code> is false,
<a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> will be left untouched meaning the <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>
method of the superclass will be used (if the superclass is
<a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>, this means it will fall back to id-based hashing).</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">frozen</span></code>: If true (the default is False), assigning to fields will
generate an exception.  This emulates read-only frozen instances.  If
<a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> or <a class="reference internal" href="../reference/datamodel.html#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delattr__()</span></code></a> is defined in the class, then
<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> is raised.  See the discussion below.</p>
<p><code class="docutils literal notranslate"><span class="pre">field</span></code>s may optionally specify a default value, using normal
Python syntax:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="n">a</span><span class="p">:</span> <span class="nb">int</span>       <span class="c1"># &#39;a&#39; has no default value</span>
    <span class="n">b</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span>   <span class="c1"># assign a default value for &#39;b&#39;</span>
</pre></div>
<p>In this example, both <code class="docutils literal notranslate"><span class="pre">a</span></code> and <code class="docutils literal notranslate"><span class="pre">b</span></code> will be included in the added
<a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, which will be defined as:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">a</span><span class="p">:</span> <span class="nb">int</span><span class="p">,</span> <span class="n">b</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span><span class="p">):</span>
</pre></div>
<p><a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> will be raised if a field without a default value
follows a field with a default value.  This is true either when this
occurs in a single class, or as a result of class inheritance.</p>
<dl class="function">
<dt id="dataclasses.field">
<code class="descclassname">dataclasses.</code><code class="descname">field</code><span class="sig-paren">(</span><em>*</em>, <em>default=MISSING</em>, <em>default_factory=MISSING</em>, <em>repr=True</em>, <em>hash=None</em>, <em>init=True</em>, <em>compare=True</em>, <em>metadata=None</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.field" title="本定義的永久連結">¶</a></dt>
<dd><p>For common and simple use cases, no other functionality is
required.  There are, however, some dataclass features that
require additional per-field information.  To satisfy this need for
additional information, you can replace the default field value
with a call to the provided <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate"><span class="pre">field()</span></code></a> function.  For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="n">mylist</span><span class="p">:</span> <span class="n">List</span><span class="p">[</span><span class="nb">int</span><span class="p">]</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="nb">list</span><span class="p">)</span>
<span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="n">c</span><span class="o">.</span><span class="n">mylist</span> <span class="o">+=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
</pre></div>
<p>As shown above, the <code class="docutils literal notranslate"><span class="pre">MISSING</span></code> value is a sentinel object used to
detect if the <code class="docutils literal notranslate"><span class="pre">default</span></code> and <code class="docutils literal notranslate"><span class="pre">default_factory</span></code> parameters are
provided.  This sentinel is used because <code class="docutils literal notranslate"><span class="pre">None</span></code> is a valid value
for <code class="docutils literal notranslate"><span class="pre">default</span></code>.  No code should directly use the <code class="docutils literal notranslate"><span class="pre">MISSING</span></code>
<p>The parameters to <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate"><span class="pre">field()</span></code></a> are:</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">default</span></code>: If provided, this will be the default value for this
field.  This is needed because the <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-meth docutils literal notranslate"><span class="pre">field()</span></code></a> call itself
replaces the normal position of the default value.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">default_factory</span></code>: If provided, it must be a zero-argument
callable that will be called when a default value is needed for
this field.  Among other purposes, this can be used to specify
fields with mutable default values, as discussed below.  It is an
error to specify both <code class="docutils literal notranslate"><span class="pre">default</span></code> and <code class="docutils literal notranslate"><span class="pre">default_factory</span></code>.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">init</span></code>: If true (the default), this field is included as a
parameter to the generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">repr</span></code>: If true (the default), this field is included in the
string returned by the generated <a class="reference internal" href="../reference/datamodel.html#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> method.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">compare</span></code>: If true (the default), this field is included in the
generated equality and comparison methods (<a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a>,
<a class="reference internal" href="../reference/datamodel.html#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__gt__()</span></code></a>, et al.).</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">hash</span></code>: This can be a bool or <code class="docutils literal notranslate"><span class="pre">None</span></code>.  If true, this field is
included in the generated <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method.  If <code class="docutils literal notranslate"><span class="pre">None</span></code> (the
default), use the value of <code class="docutils literal notranslate"><span class="pre">compare</span></code>: this would normally be
the expected behavior.  A field should be considered in the hash
if it’s used for comparisons.  Setting this value to anything
other than <code class="docutils literal notranslate"><span class="pre">None</span></code> is discouraged.</p>
<p>One possible reason to set <code class="docutils literal notranslate"><span class="pre">hash=False</span></code> but <code class="docutils literal notranslate"><span class="pre">compare=True</span></code>
would be if a field is expensive to compute a hash value for,
that field is needed for equality testing, and there are other
fields that contribute to the type’s hash value.  Even if a field
is excluded from the hash, it will still be used for comparisons.</p>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">metadata</span></code>: This can be a mapping or None. None is treated as
an empty dict.  This value is wrapped in
<a class="reference internal" href="types.html#types.MappingProxyType" title="types.MappingProxyType"><code class="xref py py-func docutils literal notranslate"><span class="pre">MappingProxyType()</span></code></a> to make it read-only, and exposed
on the <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate"><span class="pre">Field</span></code></a> object. It is not used at all by Data
Classes, and is provided as a third-party extension mechanism.
Multiple third-parties can each have their own key, to use as a
namespace in the metadata.</p>
<p>If the default value of a field is specified by a call to
<a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate"><span class="pre">field()</span></code></a>, then the class attribute for this field will be
replaced by the specified <code class="docutils literal notranslate"><span class="pre">default</span></code> value.  If no <code class="docutils literal notranslate"><span class="pre">default</span></code> is
provided, then the class attribute will be deleted.  The intent is
that after the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> decorator runs, the class
attributes will all contain the default values for the fields, just
as if the default value itself were specified.  For example,
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="n">x</span><span class="p">:</span> <span class="nb">int</span>
    <span class="n">y</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="nb">repr</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
    <span class="n">z</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="nb">repr</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">10</span><span class="p">)</span>
    <span class="n">t</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">20</span>
</pre></div>
<p>The class attribute <code class="docutils literal notranslate"><span class="pre">C.z</span></code> will be <code class="docutils literal notranslate"><span class="pre">10</span></code>, the class attribute
<code class="docutils literal notranslate"><span class="pre">C.t</span></code> will be <code class="docutils literal notranslate"><span class="pre">20</span></code>, and the class attributes <code class="docutils literal notranslate"><span class="pre">C.x</span></code> and
<code class="docutils literal notranslate"><span class="pre">C.y</span></code> will not be set.</p>
<dl class="class">
<dt id="dataclasses.Field">
<em class="property">class </em><code class="descclassname">dataclasses.</code><code class="descname">Field</code><a class="headerlink" href="#dataclasses.Field" title="本定義的永久連結">¶</a></dt>
<dd><p><a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate"><span class="pre">Field</span></code></a> objects describe each defined field. These objects
are created internally, and are returned by the <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate"><span class="pre">fields()</span></code></a>
module-level method (see below).  Users should never instantiate a
<a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate"><span class="pre">Field</span></code></a> object directly.  Its documented attributes are:</p>
<blockquote>
<div><ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">name</span></code>: The name of the field.</li>
<li><code class="docutils literal notranslate"><span class="pre">type</span></code>: The type of the field.</li>
<li><code class="docutils literal notranslate"><span class="pre">default</span></code>, <code class="docutils literal notranslate"><span class="pre">default_factory</span></code>, <code class="docutils literal notranslate"><span class="pre">init</span></code>, <code class="docutils literal notranslate"><span class="pre">repr</span></code>, <code class="docutils literal notranslate"><span class="pre">hash</span></code>,
<code class="docutils literal notranslate"><span class="pre">compare</span></code>, and <code class="docutils literal notranslate"><span class="pre">metadata</span></code> have the identical meaning and
values as they do in the <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate"><span class="pre">field()</span></code></a> declaration.</li>
</div></blockquote>
<p>Other attributes may exist, but they are private and must not be
inspected or relied on.</p>
<dl class="function">
<dt id="dataclasses.fields">
<code class="descclassname">dataclasses.</code><code class="descname">fields</code><span class="sig-paren">(</span><em>class_or_instance</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.fields" title="本定義的永久連結">¶</a></dt>
<dd><p>Returns a tuple of <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate"><span class="pre">Field</span></code></a> objects that define the fields for this
dataclass.  Accepts either a dataclass, or an instance of a dataclass.
Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> if not passed a dataclass or instance of one.
Does not return pseudo-fields which are <code class="docutils literal notranslate"><span class="pre">ClassVar</span></code> or <code class="docutils literal notranslate"><span class="pre">InitVar</span></code>.</p>
<dl class="function">
<dt id="dataclasses.asdict">
<code class="descclassname">dataclasses.</code><code class="descname">asdict</code><span class="sig-paren">(</span><em>instance</em>, <em>*</em>, <em>dict_factory=dict</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.asdict" title="本定義的永久連結">¶</a></dt>
<dd><p>Converts the dataclass <code class="docutils literal notranslate"><span class="pre">instance</span></code> to a dict (by using the
factory function <code class="docutils literal notranslate"><span class="pre">dict_factory</span></code>).  Each dataclass is converted
to a dict of its fields, as <code class="docutils literal notranslate"><span class="pre">name:</span> <span class="pre">value</span></code> pairs.  dataclasses, dicts,
lists, and tuples are recursed into.  For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">Point</span><span class="p">:</span>
     <span class="n">x</span><span class="p">:</span> <span class="nb">int</span>
     <span class="n">y</span><span class="p">:</span> <span class="nb">int</span>
<span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
     <span class="n">mylist</span><span class="p">:</span> <span class="n">List</span><span class="p">[</span><span class="n">Point</span><span class="p">]</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">Point</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">asdict</span><span class="p">(</span><span class="n">p</span><span class="p">)</span> <span class="o">==</span> <span class="p">{</span><span class="s1">&#39;x&#39;</span><span class="p">:</span> <span class="mi">10</span><span class="p">,</span> <span class="s1">&#39;y&#39;</span><span class="p">:</span> <span class="mi">20</span><span class="p">}</span>
<span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">([</span><span class="n">Point</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">),</span> <span class="n">Point</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">4</span><span class="p">)])</span>
<span class="k">assert</span> <span class="n">asdict</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="o">==</span> <span class="p">{</span><span class="s1">&#39;mylist&#39;</span><span class="p">:</span> <span class="p">[{</span><span class="s1">&#39;x&#39;</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span> <span class="s1">&#39;y&#39;</span><span class="p">:</span> <span class="mi">0</span><span class="p">},</span> <span class="p">{</span><span class="s1">&#39;x&#39;</span><span class="p">:</span> <span class="mi">10</span><span class="p">,</span> <span class="s1">&#39;y&#39;</span><span class="p">:</span> <span class="mi">4</span><span class="p">}]}</span>
</pre></div>
<p>Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> if <code class="docutils literal notranslate"><span class="pre">instance</span></code> is not a dataclass instance.</p>
<dl class="function">
<dt id="dataclasses.astuple">
<code class="descclassname">dataclasses.</code><code class="descname">astuple</code><span class="sig-paren">(</span><em>*</em>, <em>tuple_factory=tuple</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.astuple" title="本定義的永久連結">¶</a></dt>
<dd><p>Converts the dataclass <code class="docutils literal notranslate"><span class="pre">instance</span></code> to a tuple (by using the
factory function <code class="docutils literal notranslate"><span class="pre">tuple_factory</span></code>).  Each dataclass is converted
to a tuple of its field values.  dataclasses, dicts, lists, and
tuples are recursed into.</p>
<p>Continuing from the previous example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">assert</span> <span class="n">astuple</span><span class="p">(</span><span class="n">p</span><span class="p">)</span> <span class="o">==</span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">astuple</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="o">==</span> <span class="p">([(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">),</span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">4</span><span class="p">)],)</span>
</pre></div>
<p>Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> if <code class="docutils literal notranslate"><span class="pre">instance</span></code> is not a dataclass instance.</p>
<dl class="function">
<dt id="dataclasses.make_dataclass">
<code class="descclassname">dataclasses.</code><code class="descname">make_dataclass</code><span class="sig-paren">(</span><em>cls_name</em>, <em>fields</em>, <em>*</em>, <em>bases=()</em>, <em>namespace=None</em>, <em>init=True</em>, <em>repr=True</em>, <em>eq=True</em>, <em>order=False</em>, <em>unsafe_hash=False</em>, <em>frozen=False</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.make_dataclass" title="本定義的永久連結">¶</a></dt>
<dd><p>Creates a new dataclass with name <code class="docutils literal notranslate"><span class="pre">cls_name</span></code>, fields as defined
in <code class="docutils literal notranslate"><span class="pre">fields</span></code>, base classes as given in <code class="docutils literal notranslate"><span class="pre">bases</span></code>, and initialized
with a namespace as given in <code class="docutils literal notranslate"><span class="pre">namespace</span></code>.  <code class="docutils literal notranslate"><span class="pre">fields</span></code> is an
iterable whose elements are each either <code class="docutils literal notranslate"><span class="pre">name</span></code>, <code class="docutils literal notranslate"><span class="pre">(name,</span> <span class="pre">type)</span></code>,
or <code class="docutils literal notranslate"><span class="pre">(name,</span> <span class="pre">type,</span> <span class="pre">Field)</span></code>.  If just <code class="docutils literal notranslate"><span class="pre">name</span></code> is supplied,
<code class="docutils literal notranslate"><span class="pre">typing.Any</span></code> is used for <code class="docutils literal notranslate"><span class="pre">type</span></code>.  The values of <code class="docutils literal notranslate"><span class="pre">init</span></code>,
<code class="docutils literal notranslate"><span class="pre">repr</span></code>, <code class="docutils literal notranslate"><span class="pre">eq</span></code>, <code class="docutils literal notranslate"><span class="pre">order</span></code>, <code class="docutils literal notranslate"><span class="pre">unsafe_hash</span></code>, and <code class="docutils literal notranslate"><span class="pre">frozen</span></code> have
the same meaning as they do in <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a>.</p>
<p>This function is not strictly required, because any Python
mechanism for creating a new class with <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> can
then apply the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> function to convert that class to
a dataclass.  This function is provided as a convenience.  For
example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">C</span> <span class="o">=</span> <span class="n">make_dataclass</span><span class="p">(</span><span class="s1">&#39;C&#39;</span><span class="p">,</span>
                   <span class="p">[(</span><span class="s1">&#39;x&#39;</span><span class="p">,</span> <span class="nb">int</span><span class="p">),</span>
                     <span class="s1">&#39;y&#39;</span><span class="p">,</span>
                    <span class="p">(</span><span class="s1">&#39;z&#39;</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="n">field</span><span class="p">(</span><span class="n">default</span><span class="o">=</span><span class="mi">5</span><span class="p">))],</span>
                   <span class="n">namespace</span><span class="o">=</span><span class="p">{</span><span class="s1">&#39;add_one&#39;</span><span class="p">:</span> <span class="k">lambda</span> <span class="bp">self</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">+</span> <span class="mi">1</span><span class="p">})</span>
</pre></div>
<p>Is equivalent to:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="n">x</span><span class="p">:</span> <span class="nb">int</span>
    <span class="n">y</span><span class="p">:</span> <span class="s1">&#39;typing.Any&#39;</span>
    <span class="n">z</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">5</span>
    <span class="k">def</span> <span class="nf">add_one</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">+</span> <span class="mi">1</span>
</pre></div>
<dl class="function">
<dt id="dataclasses.replace">
<code class="descclassname">dataclasses.</code><code class="descname">replace</code><span class="sig-paren">(</span><em>instance</em>, <em>**changes</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.replace" title="本定義的永久連結">¶</a></dt>
<dd><p>Creates a new object of the same type of <code class="docutils literal notranslate"><span class="pre">instance</span></code>, replacing
fields with values from <code class="docutils literal notranslate"><span class="pre">changes</span></code>.  If <code class="docutils literal notranslate"><span class="pre">instance</span></code> is not a Data
Class, raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.  If values in <code class="docutils literal notranslate"><span class="pre">changes</span></code> do not
specify fields, raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p>
<p>The newly returned object is created by calling the <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a>
method of the dataclass.  This ensures that
<code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code>, if present, is also called.</p>
<p>Init-only variables without default values, if any exist, must be
specified on the call to <a class="reference internal" href="#dataclasses.replace" title="dataclasses.replace"><code class="xref py py-func docutils literal notranslate"><span class="pre">replace()</span></code></a> so that they can be passed to
<a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code>.</p>
<p>It is an error for <code class="docutils literal notranslate"><span class="pre">changes</span></code> to contain any fields that are
defined as having <code class="docutils literal notranslate"><span class="pre">init=False</span></code>.  A <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> will be raised
in this case.</p>
<p>Be forewarned about how <code class="docutils literal notranslate"><span class="pre">init=False</span></code> fields work during a call to
<a class="reference internal" href="#dataclasses.replace" title="dataclasses.replace"><code class="xref py py-func docutils literal notranslate"><span class="pre">replace()</span></code></a>.  They are not copied from the source object, but
rather are initialized in <code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code>, if they’re
initialized at all.  It is expected that <code class="docutils literal notranslate"><span class="pre">init=False</span></code> fields will
be rarely and judiciously used.  If they are used, it might be wise
to have alternate class constructors, or perhaps a custom
<code class="docutils literal notranslate"><span class="pre">replace()</span></code> (or similarly named) method which handles instance
copying.</p>
<dl class="function">
<dt id="dataclasses.is_dataclass">
<code class="descclassname">dataclasses.</code><code class="descname">is_dataclass</code><span class="sig-paren">(</span><em>class_or_instance</em><span class="sig-paren">)</span><a class="headerlink" href="#dataclasses.is_dataclass" title="本定義的永久連結">¶</a></dt>
<dd><p>Returns True if its parameter is a dataclass or an instance of one,
otherwise returns False.</p>
<p>If you need to know if a class is an instance of a dataclass (and
not a dataclass itself), then add a further check for <code class="docutils literal notranslate"><span class="pre">not</span>
<span class="pre">isinstance(obj,</span> <span class="pre">type)</span></code>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">is_dataclass_instance</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>
    <span class="k">return</span> <span class="n">is_dataclass</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span> <span class="ow">and</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="nb">type</span><span class="p">)</span>
</pre></div>
<div class="section" id="post-init-processing">
<h2>30.6.2. Post-init processing<a class="headerlink" href="#post-init-processing" title="本標題的永久連結">¶</a></h2>
<p>The generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> code will call a method named
<code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code>, if <code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code> is defined on the
class.  It will normally be called as <code class="docutils literal notranslate"><span class="pre">self.__post_init__()</span></code>.
However, if any <code class="docutils literal notranslate"><span class="pre">InitVar</span></code> fields are defined, they will also be
passed to <code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code> in the order they were defined in the
class.  If no <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method is generated, then
<code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code> will not automatically be called.</p>
<p>Among other uses, this allows for initializing field values that
depend on one or more other fields.  For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="n">a</span><span class="p">:</span> <span class="nb">float</span>
    <span class="n">b</span><span class="p">:</span> <span class="nb">float</span>
    <span class="n">c</span><span class="p">:</span> <span class="nb">float</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">init</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
    <span class="k">def</span> <span class="nf">__post_init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">c</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">a</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">b</span>
</pre></div>
<p>See the section below on init-only variables for ways to pass
parameters to <code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code>.  Also see the warning about how
<a class="reference internal" href="#dataclasses.replace" title="dataclasses.replace"><code class="xref py py-func docutils literal notranslate"><span class="pre">replace()</span></code></a> handles <code class="docutils literal notranslate"><span class="pre">init=False</span></code> fields.</p>
<div class="section" id="class-variables">
<h2>30.6.3. Class variables<a class="headerlink" href="#class-variables" title="本標題的永久連結">¶</a></h2>
<p>One of two places where <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> actually inspects the type
of a field is to determine if a field is a class variable as defined
in <span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0526"><strong>PEP 526</strong></a>.  It does this by checking if the type of the field is
<code class="docutils literal notranslate"><span class="pre">typing.ClassVar</span></code>.  If a field is a <code class="docutils literal notranslate"><span class="pre">ClassVar</span></code>, it is excluded
from consideration as a field and is ignored by the dataclass
mechanisms.  Such <code class="docutils literal notranslate"><span class="pre">ClassVar</span></code> pseudo-fields are not returned by the
module-level <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate"><span class="pre">fields()</span></code></a> function.</p>
<div class="section" id="init-only-variables">
<h2>30.6.4. Init-only variables<a class="headerlink" href="#init-only-variables" title="本標題的永久連結">¶</a></h2>
<p>The other place where <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">dataclass()</span></code></a> inspects a type annotation is to
determine if a field is an init-only variable.  It does this by seeing
if the type of a field is of type <code class="docutils literal notranslate"><span class="pre">dataclasses.InitVar</span></code>.  If a field
is an <code class="docutils literal notranslate"><span class="pre">InitVar</span></code>, it is considered a pseudo-field called an init-only
field.  As it is not a true field, it is not returned by the
module-level <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate"><span class="pre">fields()</span></code></a> function.  Init-only fields are added as
parameters to the generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, and are passed to
the optional <code class="xref py py-meth docutils literal notranslate"><span class="pre">__post_init__()</span></code> method.  They are not otherwise used
by dataclasses.</p>
<p>For example, suppose a field will be initialzed from a database, if a
value is not provided when creating the class:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="n">i</span><span class="p">:</span> <span class="nb">int</span>
    <span class="n">j</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="kc">None</span>
    <span class="n">database</span><span class="p">:</span> <span class="n">InitVar</span><span class="p">[</span><span class="n">DatabaseType</span><span class="p">]</span> <span class="o">=</span> <span class="kc">None</span>
    <span class="k">def</span> <span class="nf">__post_init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">database</span><span class="p">):</span>
        <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">j</span> <span class="ow">is</span> <span class="kc">None</span> <span class="ow">and</span> <span class="n">database</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">j</span> <span class="o">=</span> <span class="n">database</span><span class="o">.</span><span class="n">lookup</span><span class="p">(</span><span class="s1">&#39;j&#39;</span><span class="p">)</span>
<span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="n">database</span><span class="o">=</span><span class="n">my_database</span><span class="p">)</span>
</pre></div>
<p>In this case, <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate"><span class="pre">fields()</span></code></a> will return <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate"><span class="pre">Field</span></code></a> objects for <code class="docutils literal notranslate"><span class="pre">i</span></code> and
<code class="docutils literal notranslate"><span class="pre">j</span></code>, but not for <code class="docutils literal notranslate"><span class="pre">database</span></code>.</p>
<div class="section" id="frozen-instances">
<h2>30.6.5. Frozen instances<a class="headerlink" href="#frozen-instances" title="本標題的永久連結">¶</a></h2>
<p>It is not possible to create truly immutable Python objects.  However,
by passing <code class="docutils literal notranslate"><span class="pre">frozen=True</span></code> to the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-meth docutils literal notranslate"><span class="pre">dataclass()</span></code></a> decorator you can
emulate immutability.  In that case, dataclasses will add
<a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delattr__()</span></code></a> methods to the class.  These
methods will raise a <a class="reference internal" href="#dataclasses.FrozenInstanceError" title="dataclasses.FrozenInstanceError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">FrozenInstanceError</span></code></a> when invoked.</p>
<p>There is a tiny performance penalty when using <code class="docutils literal notranslate"><span class="pre">frozen=True</span></code>:
<a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> cannot use simple assignment to initialize fields, and
must use <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__setattr__()</span></code></a>.</p>
<div class="section" id="inheritance">
<h2>30.6.6. Inheritance<a class="headerlink" href="#inheritance" title="本標題的永久連結">¶</a></h2>
<p>When the dataclass is being created by the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-meth docutils literal notranslate"><span class="pre">dataclass()</span></code></a> decorator,
it looks through all of the class’s base classes in reverse MRO (that
is, starting at <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>) and, for each dataclass that it finds,
adds the fields from that base class to an ordered mapping of fields.
After all of the base class fields are added, it adds its own fields
to the ordered mapping.  All of the generated methods will use this
combined, calculated ordered mapping of fields.  Because the fields
are in insertion order, derived classes override base classes.  An
example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">Base</span><span class="p">:</span>
    <span class="n">x</span><span class="p">:</span> <span class="n">Any</span> <span class="o">=</span> <span class="mf">15.0</span>
    <span class="n">y</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span>
<span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
    <span class="n">z</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">10</span>
    <span class="n">x</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">15</span>
</pre></div>
<p>The final list of fields is, in order, <code class="docutils literal notranslate"><span class="pre">x</span></code>, <code class="docutils literal notranslate"><span class="pre">y</span></code>, <code class="docutils literal notranslate"><span class="pre">z</span></code>.  The final
type of <code class="docutils literal notranslate"><span class="pre">x</span></code> is <code class="docutils literal notranslate"><span class="pre">int</span></code>, as specified in class <code class="docutils literal notranslate"><span class="pre">C</span></code>.</p>
<p>The generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method for <code class="docutils literal notranslate"><span class="pre">C</span></code> will look like:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">15</span><span class="p">,</span> <span class="n">y</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="n">z</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">10</span><span class="p">):</span>
</pre></div>
<div class="section" id="default-factory-functions">
<h2>30.6.7. Default factory functions<a class="headerlink" href="#default-factory-functions" title="本標題的永久連結">¶</a></h2>
<blockquote>
<div><p>If a <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate"><span class="pre">field()</span></code></a> specifies a <code class="docutils literal notranslate"><span class="pre">default_factory</span></code>, it is called with
zero arguments when a default value for the field is needed.  For
example, to create a new instance of a list, use:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">mylist</span><span class="p">:</span> <span class="nb">list</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="nb">list</span><span class="p">)</span>
</pre></div>
<p>If a field is excluded from <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> (using <code class="docutils literal notranslate"><span class="pre">init=False</span></code>)
and the field also specifies <code class="docutils literal notranslate"><span class="pre">default_factory</span></code>, then the default
factory function will always be called from the generated
<a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> function.  This happens because there is no other
way to give the field an initial value.</p>
</div></blockquote>
<div class="section" id="mutable-default-values">
<h2>30.6.8. Mutable default values<a class="headerlink" href="#mutable-default-values" title="本標題的永久連結">¶</a></h2>
<blockquote>
<div><p>Python stores default member variable values in class attributes.
Consider this example, not using dataclasses:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
    <span class="n">x</span> <span class="o">=</span> <span class="p">[]</span>
    <span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">element</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">x</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">element</span><span class="p">)</span>
<span class="n">o1</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="n">o2</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="n">o1</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">o2</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">o1</span><span class="o">.</span><span class="n">x</span> <span class="o">==</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">]</span>
<span class="k">assert</span> <span class="n">o1</span><span class="o">.</span><span class="n">x</span> <span class="ow">is</span> <span class="n">o2</span><span class="o">.</span><span class="n">x</span>
</pre></div>
<p>Note that the two instances of class <code class="docutils literal notranslate"><span class="pre">C</span></code> share the same class
variable <code class="docutils literal notranslate"><span class="pre">x</span></code>, as expected.</p>
<p>Using dataclasses, <em>if</em> this code was valid:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">D</span><span class="p">:</span>
    <span class="n">x</span><span class="p">:</span> <span class="n">List</span> <span class="o">=</span> <span class="p">[]</span>
    <span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">element</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">+=</span> <span class="n">element</span>
</pre></div>
<p>it would generate code similar to:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">D</span><span class="p">:</span>
    <span class="n">x</span> <span class="o">=</span> <span class="p">[]</span>
    <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="o">=</span><span class="n">x</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="n">x</span>
    <span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">element</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">+=</span> <span class="n">element</span>
<span class="k">assert</span> <span class="n">D</span><span class="p">()</span><span class="o">.</span><span class="n">x</span> <span class="ow">is</span> <span class="n">D</span><span class="p">()</span><span class="o">.</span><span class="n">x</span>
</pre></div>
<p>This has the same issue as the original example using class <code class="docutils literal notranslate"><span class="pre">C</span></code>.
That is, two instances of class <code class="docutils literal notranslate"><span class="pre">D</span></code> that do not specify a value for
<code class="docutils literal notranslate"><span class="pre">x</span></code> when creating a class instance will share the same copy of
<code class="docutils literal notranslate"><span class="pre">x</span></code>.  Because dataclasses just use normal Python class creation
they also share this behavior.  There is no general way for Data
Classes to detect this condition.  Instead, dataclasses will raise a
<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> if it detects a default parameter of type <code class="docutils literal notranslate"><span class="pre">list</span></code>,
<code class="docutils literal notranslate"><span class="pre">dict</span></code>, or <code class="docutils literal notranslate"><span class="pre">set</span></code>.  This is a partial solution, but it does protect
against many common errors.</p>
<p>Using default factory functions is a way to create new instances of
mutable types as default values for fields:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@dataclass</span>
<span class="k">class</span> <span class="nc">D</span><span class="p">:</span>
    <span class="n">x</span><span class="p">:</span> <span class="nb">list</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="nb">list</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">D</span><span class="p">()</span><span class="o">.</span><span class="n">x</span> <span class="ow">is</span> <span class="ow">not</span> <span class="n">D</span><span class="p">()</span><span class="o">.</span><span class="n">x</span>
</pre></div>
</div></blockquote>
<div class="section" id="exceptions">
<h2>30.6.9. Exceptions<a class="headerlink" href="#exceptions" title="本標題的永久連結">¶</a></h2>
<dl class="exception">
<dt id="dataclasses.FrozenInstanceError">
<em class="property">exception </em><code class="descclassname">dataclasses.</code><code class="descname">FrozenInstanceError</code><a class="headerlink" href="#dataclasses.FrozenInstanceError" title="本定義的永久連結">¶</a></dt>
<dd><p>Raised when an implicitly defined <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> or
<a class="reference internal" href="../reference/datamodel.html#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delattr__()</span></code></a> is called on a dataclass which was defined with
<code class="docutils literal notranslate"><span class="pre">frozen=True</span></code>.</p>
          </div>
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  <h3><a href="../contents.html">目錄</a></h3>
<li><a class="reference internal" href="#">30.6. <code class="docutils literal notranslate"><span class="pre">dataclasses</span></code> — Data Classes</a><ul>
<li><a class="reference internal" href="#module-level-decorators-classes-and-functions">30.6.1. Module-level decorators, classes, and functions</a></li>
<li><a class="reference internal" href="#post-init-processing">30.6.2. Post-init processing</a></li>
<li><a class="reference internal" href="#class-variables">30.6.3. Class variables</a></li>
<li><a class="reference internal" href="#init-only-variables">30.6.4. Init-only variables</a></li>
<li><a class="reference internal" href="#frozen-instances">30.6.5. Frozen instances</a></li>
<li><a class="reference internal" href="#inheritance">30.6.6. Inheritance</a></li>
<li><a class="reference internal" href="#default-factory-functions">30.6.7. Default factory functions</a></li>
<li><a class="reference internal" href="#mutable-default-values">30.6.8. Mutable default values</a></li>
<li><a class="reference internal" href="#exceptions">30.6.9. Exceptions</a></li>
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