Adds better type narrowing docs (#11088)

sobolevn · hauntsaninja · web-flow · commit b3ff2a683bd4 · 2021-09-19T16:13:13.000-07:00
Co-authored-by: Shantanu &lt;12621235+hauntsaninja@users.noreply.github.com&gt;
diff --git a/docs/source/casts.rst b/docs/source/casts.rst
diff --git a/docs/source/common_issues.rst b/docs/source/common_issues.rst
@@ -370,7 +370,7 @@ Complex type tests
 Mypy can usually infer the types correctly when using :py:func:`isinstance <isinstance>`,
 :py:func:`issubclass <issubclass>`,
 or ``type(obj) is some_class`` type tests,
-and even user-defined type guards,
+and even :ref:`user-defined type guards <type-guards>`,
 but for other kinds of checks you may need to add an
 explicit type cast:
 
@@ -385,7 +385,7 @@ explicit type cast:
 
       found = a[index]  # Has `object` type, despite the fact that we know it is `str`
       return cast(str, found)  # So, we need an explicit cast to make mypy happy
-      
+
 Alternatively, you can use ``assert`` statement together with some
 of the supported type inference techniques:
 
@@ -729,6 +729,8 @@ not necessary:
         def test(self, t: List[int]) -> Sequence[str]:  # type: ignore[override]
             ...
 
+.. _unreachable:
+
 Unreachable code
 ----------------
 
diff --git a/docs/source/dynamic_typing.rst b/docs/source/dynamic_typing.rst
@@ -80,7 +80,7 @@ operations:
         n = 1  # type: int
         n = o         # Error!
 
-You can use :py:func:`~typing.cast` (see chapter :ref:`casts`) or :py:func:`isinstance` to
-go from a general type such as :py:class:`object` to a more specific
-type (subtype) such as ``int``.  :py:func:`~typing.cast` is not needed with
+You can use different :ref:`type narrowing <type-narrowing>`
+techniques to narrow :py:class:`object` to a more specific
+type (subtype) such as ``int``. Type narrowing is not needed with
 dynamically typed values (values with type ``Any``).
diff --git a/docs/source/generics.rst b/docs/source/generics.rst
@@ -562,7 +562,7 @@ non-function (e.g. ``my_decorator(1)``) will be rejected.
 Also note that the ``wrapper()`` function is not type-checked. Wrapper
 functions are typically small enough that this is not a big
 problem. This is also the reason for the :py:func:`~typing.cast` call in the
-``return`` statement in ``my_decorator()``. See :ref:`casts`.
+``return`` statement in ``my_decorator()``. See :ref:`casts <casts>`.
 
 .. _decorator-factories:
 
diff --git a/docs/source/index.rst b/docs/source/index.rst
@@ -39,7 +39,7 @@ Mypy is a static type checker for Python 3 and Python 2.7.
    protocols
    dynamic_typing
    python2
-   casts
+   type_narrowing
    duck_type_compatibility
    stubs
    generics
diff --git a/docs/source/more_types.rst b/docs/source/more_types.rst
@@ -1151,148 +1151,3 @@ section of the docs has a full description with an example, but in short, you wi
 need to give each TypedDict the same key where each value has a unique
 unique :ref:`Literal type <literal_types>`. Then, check that key to distinguish
 between your TypedDicts.
-
-
-User-Defined Type Guards
-************************
-
-Mypy supports User-Defined Type Guards
-(:pep:`647`).
-
-A type guard is a way for programs to influence conditional
-type narrowing employed by a type checker based on runtime checks.
-
-Basically, a ``TypeGuard`` is a "smart" alias for a ``bool`` type.
-Let's have a look at the regular ``bool`` example:
-
-.. code-block:: python
-
-  from typing import List
-
-  def is_str_list(val: List[object]) -> bool:
-    """Determines whether all objects in the list are strings"""
-    return all(isinstance(x, str) for x in val)
-
-  def func1(val: List[object]) -> None:
-      if is_str_list(val):
-          reveal_type(val)  # Reveals List[object]
-          print(" ".join(val)) # Error: incompatible type
-
-The same example with ``TypeGuard``:
-
-.. code-block:: python
-
-  from typing import List
-  from typing import TypeGuard  # use `typing_extensions` for Python 3.9 and below
-
-  def is_str_list(val: List[object]) -> TypeGuard[List[str]]:
-      """Determines whether all objects in the list are strings"""
-      return all(isinstance(x, str) for x in val)
-
-  def func1(val: List[object]) -> None:
-      if is_str_list(val):
-          reveal_type(val)  # List[str]
-          print(" ".join(val)) # ok
-
-How does it work? ``TypeGuard`` narrows the first function argument (``val``)
-to the type specified as the first type parameter (``List[str]``).
-
-.. note::
-
-  Narrowing is
-  `not strict <https://www.python.org/dev/peps/pep-0647/#enforcing-strict-narrowing>`_.
-  For example, you can narrow ``str`` to ``int``:
-
-  .. code-block:: python
-
-    def f(value: str) -> TypeGuard[int]:
-        return True
-
-  Note: since strict narrowing is not enforced, it's easy
-  to break type safety.
-
-  However, there are many ways a determined or uninformed developer can
-  subvert type safety -- most commonly by using cast or Any.
-  If a Python developer takes the time to learn about and implement
-  user-defined type guards within their code,
-  it is safe to assume that they are interested in type safety
-  and will not write their type guard functions in a way
-  that will undermine type safety or produce nonsensical results.
-
-Generic TypeGuards
-------------------
-
-``TypeGuard`` can also work with generic types:
-
-.. code-block:: python
-
-  from typing import Tuple, TypeVar
-  from typing import TypeGuard  # use `typing_extensions` for `python<3.10`
-
-  _T = TypeVar("_T")
-
-  def is_two_element_tuple(val: Tuple[_T, ...]) -> TypeGuard[Tuple[_T, _T]]:
-      return len(val) == 2
-
-  def func(names: Tuple[str, ...]):
-      if is_two_element_tuple(names):
-          reveal_type(names)  # Tuple[str, str]
-      else:
-          reveal_type(names)  # Tuple[str, ...]
-
-Typeguards with parameters
---------------------------
-
-Type guard functions can accept extra arguments:
-
-.. code-block:: python
-
-  from typing import Type, Set, TypeVar
-  from typing import TypeGuard  # use `typing_extensions` for `python<3.10`
-
-  _T = TypeVar("_T")
-
-  def is_set_of(val: Set[Any], type: Type[_T]) -> TypeGuard[Set[_T]]:
-      return all(isinstance(x, type) for x in val)
-
-  items: Set[Any]
-  if is_set_of(items, str):
-      reveal_type(items)  # Set[str]
-
-TypeGuards as methods
----------------------
-
- A method can also serve as the ``TypeGuard``:
-
-.. code-block:: python
-
-  class StrValidator:
-      def is_valid(self, instance: object) -> TypeGuard[str]:
-          return isinstance(instance, str)
-
-  def func(to_validate: object) -> None:
-      if StrValidator().is_valid(to_validate):
-          reveal_type(to_validate)  # Revealed type is "builtins.str"
-
-.. note::
-
-  Note, that ``TypeGuard``
-  `does not narrow <https://www.python.org/dev/peps/pep-0647/#narrowing-of-implicit-self-and-cls-parameters>`_
-  types of ``self`` or ``cls`` implicit arguments.
-
-  If narrowing of ``self`` or ``cls`` is required,
-  the value can be passed as an explicit argument to a type guard function:
-
-  .. code-block:: python
-
-    class Parent:
-        def method(self) -> None:
-            reveal_type(self)  # Revealed type is "Parent"
-            if is_child(self):
-                reveal_type(self)  # Revealed type is "Child"
-
-    class Child(Parent):
-        ...
-
-    def is_child(instance: Parent) -> TypeGuard[Child]:
-        return isinstance(instance, Child)
diff --git a/docs/source/runtime_troubles.rst b/docs/source/runtime_troubles.rst
@@ -62,7 +62,7 @@ required to be valid Python syntax. For more details, see :pep:`563`.
     of forward references or generics in:
 
     * :ref:`type aliases <type-aliases>`;
-    * :ref:`casts <casts>`;
+    * :ref:`type narrowing <type-narrowing>`;
     * type definitions (see :py:class:`~typing.TypeVar`, :py:func:`~typing.NewType`, :py:class:`~typing.NamedTuple`);
     * base classes.
 
diff --git a/docs/source/type_narrowing.rst b/docs/source/type_narrowing.rst
