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rhettingerrhettinger
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bpo-32320: Add default value support to collections.namedtuple() (#4859)
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Lines changed: 93 additions & 4 deletions

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‎Doc/library/collections.rst‎

Lines changed: 24 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -782,7 +782,7 @@ Named tuples assign meaning to each position in a tuple and allow for more reada
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self-documenting code. They can be used wherever regular tuples are used, and
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they add the ability to access fields by name instead of position index.
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.. function:: namedtuple(typename, field_names, *, rename=False, module=None)
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.. function:: namedtuple(typename, field_names, *, rename=False, defaults=None, module=None)
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Returns a new tuple subclass named *typename*. The new subclass is used to
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create tuple-like objects that have fields accessible by attribute lookup as
@@ -805,6 +805,13 @@ they add the ability to access fields by name instead of position index.
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converted to ``['abc', '_1', 'ghi', '_3']``, eliminating the keyword
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``def`` and the duplicate fieldname ``abc``.
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*defaults* can be ``None`` or an :term:`iterable` of default values.
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Since fields with a default value must come after any fields without a
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default, the *defaults* are applied to the rightmost parameters. For
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example, if the fieldnames are ``['x', 'y', 'z']`` and the defaults are
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``(1, 2)``, then ``x`` will be a required argument, ``y`` will default to
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``1``, and ``z`` will default to ``2``.
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If *module* is defined, the ``__module__`` attribute of the named tuple is
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set to that value.
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@@ -824,6 +831,10 @@ they add the ability to access fields by name instead of position index.
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.. versionchanged:: 3.7
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Remove the *verbose* parameter and the :attr:`_source` attribute.
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.. versionchanged:: 3.7
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Added the *defaults* parameter and the :attr:`_field_defaults`
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attribute.
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.. doctest::
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:options: +NORMALIZE_WHITESPACE
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@@ -911,6 +922,18 @@ field names, the method and attribute names start with an underscore.
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>>> Pixel(11, 22, 128, 255, 0)
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Pixel(x=11, y=22, red=128, green=255, blue=0)
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.. attribute:: somenamedtuple._fields_defaults
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Dictionary mapping field names to default values.
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.. doctest::
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>>> Account = namedtuple('Account', ['type', 'balance'], defaults=[0])
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>>> Account._fields_defaults
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{'balance': 0}
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>>> Account('premium')
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Account(type='premium', balance=0)
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To retrieve a field whose name is stored in a string, use the :func:`getattr`
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function:
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‎Lib/collections/__init__.py‎

Lines changed: 17 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -303,7 +303,7 @@ def __eq__(self, other):
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_nt_itemgetters = {}
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306-
def namedtuple(typename, field_names, *, rename=False, module=None):
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def namedtuple(typename, field_names, *, rename=False, defaults=None, module=None):
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"""Returns a new subclass of tuple with named fields.
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>>> Point = namedtuple('Point', ['x', 'y'])
@@ -332,7 +332,8 @@ def namedtuple(typename, field_names, *, rename=False, module=None):
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if isinstance(field_names, str):
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field_names = field_names.replace(',', ' ').split()
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field_names = list(map(str, field_names))
335-
typename = str(typename)
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typename = _sys.intern(str(typename))
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if rename:
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seen = set()
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for index, name in enumerate(field_names):
@@ -342,6 +343,7 @@ def namedtuple(typename, field_names, *, rename=False, module=None):
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or name in seen):
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field_names[index] = f'_{index}'
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seen.add(name)
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for name in [typename] + field_names:
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if type(name) is not str:
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raise TypeError('Type names and field names must be strings')
@@ -351,6 +353,7 @@ def namedtuple(typename, field_names, *, rename=False, module=None):
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if _iskeyword(name):
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raise ValueError('Type names and field names cannot be a '
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f'keyword: {name!r}')
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seen = set()
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for name in field_names:
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if name.startswith('_') and not rename:
@@ -360,6 +363,14 @@ def namedtuple(typename, field_names, *, rename=False, module=None):
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raise ValueError(f'Encountered duplicate field name: {name!r}')
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seen.add(name)
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field_defaults = {}
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if defaults is not None:
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defaults = tuple(defaults)
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if len(defaults) > len(field_names):
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raise TypeError('Got more default values than field names')
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field_defaults = dict(reversed(list(zip(reversed(field_names),
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reversed(defaults)))))
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# Variables used in the methods and docstrings
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field_names = tuple(map(_sys.intern, field_names))
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num_fields = len(field_names)
@@ -372,10 +383,12 @@ def namedtuple(typename, field_names, *, rename=False, module=None):
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s = f'def __new__(_cls, {arg_list}): return _tuple_new(_cls, ({arg_list}))'
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namespace = {'_tuple_new': tuple_new, '__name__': f'namedtuple_{typename}'}
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# Note: exec() has the side-effect of interning the typename and field names
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# Note: exec() has the side-effect of interning the field names
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exec(s, namespace)
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__new__ = namespace['__new__']
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__new__.__doc__ = f'Create new instance of {typename}({arg_list})'
390+
if defaults is not None:
391+
__new__.__defaults__ = defaults
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@classmethod
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def _make(cls, iterable):
@@ -420,6 +433,7 @@ def __getnewargs__(self):
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'__doc__': f'{typename}({arg_list})',
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'__slots__': (),
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'_fields': field_names,
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'_fields_defaults': field_defaults,
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'__new__': __new__,
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'_make': _make,
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'_replace': _replace,

‎Lib/test/test_collections.py‎

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Original file line numberDiff line numberDiff line change
@@ -216,6 +216,57 @@ def test_factory(self):
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self.assertRaises(TypeError, Point._make, [11]) # catch too few args
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self.assertRaises(TypeError, Point._make, [11, 22, 33]) # catch too many args
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def test_defaults(self):
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Point = namedtuple('Point', 'x y', defaults=(10, 20)) # 2 defaults
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self.assertEqual(Point._fields_defaults, {'x': 10, 'y': 20})
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self.assertEqual(Point(1, 2), (1, 2))
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self.assertEqual(Point(1), (1, 20))
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self.assertEqual(Point(), (10, 20))
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226+
Point = namedtuple('Point', 'x y', defaults=(20,)) # 1 default
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self.assertEqual(Point._fields_defaults, {'y': 20})
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self.assertEqual(Point(1, 2), (1, 2))
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self.assertEqual(Point(1), (1, 20))
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231+
Point = namedtuple('Point', 'x y', defaults=()) # 0 defaults
232+
self.assertEqual(Point._fields_defaults, {})
233+
self.assertEqual(Point(1, 2), (1, 2))
234+
with self.assertRaises(TypeError):
235+
Point(1)
236+
237+
with self.assertRaises(TypeError): # catch too few args
238+
Point()
239+
with self.assertRaises(TypeError): # catch too many args
240+
Point(1, 2, 3)
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with self.assertRaises(TypeError): # too many defaults
242+
Point = namedtuple('Point', 'x y', defaults=(10, 20, 30))
243+
with self.assertRaises(TypeError): # non-iterable defaults
244+
Point = namedtuple('Point', 'x y', defaults=10)
245+
with self.assertRaises(TypeError): # another non-iterable default
246+
Point = namedtuple('Point', 'x y', defaults=False)
247+
248+
Point = namedtuple('Point', 'x y', defaults=None) # default is None
249+
self.assertEqual(Point._fields_defaults, {})
250+
self.assertIsNone(Point.__new__.__defaults__, None)
251+
self.assertEqual(Point(10, 20), (10, 20))
252+
with self.assertRaises(TypeError): # catch too few args
253+
Point(10)
254+
255+
Point = namedtuple('Point', 'x y', defaults=[10, 20]) # allow non-tuple iterable
256+
self.assertEqual(Point._fields_defaults, {'x': 10, 'y': 20})
257+
self.assertEqual(Point.__new__.__defaults__, (10, 20))
258+
self.assertEqual(Point(1, 2), (1, 2))
259+
self.assertEqual(Point(1), (1, 20))
260+
self.assertEqual(Point(), (10, 20))
261+
262+
Point = namedtuple('Point', 'x y', defaults=iter([10, 20])) # allow plain iterator
263+
self.assertEqual(Point._fields_defaults, {'x': 10, 'y': 20})
264+
self.assertEqual(Point.__new__.__defaults__, (10, 20))
265+
self.assertEqual(Point(1, 2), (1, 2))
266+
self.assertEqual(Point(1), (1, 20))
267+
self.assertEqual(Point(), (10, 20))
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269+
219270
@unittest.skipIf(sys.flags.optimize >= 2,
220271
"Docstrings are omitted with -O2 and above")
221272
def test_factory_doc_attr(self):

‎Misc/NEWS.d/next/Library/2017-12-14-01-36-25.bpo-32320.jwOZlr.rst‎

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@@ -0,0 +1 @@
1+
collections.namedtuple() now supports default values.

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