Minor documentation updates (#12329)

* Some minor documentation updates
* Add more discussion of exhaustiveness checking
* Update docs/source/literal_types.rst
* Simplify docs for legacy async

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

docs/source/config_file.rst

@@ -201,7 +201,7 @@ section of the command line docs.
 
     A regular expression that matches file names, directory names and paths
     which mypy should ignore while recursively discovering files to check.
-    Use forward slashes on all platforms.
+    Use forward slashes (``/``) as directory separators on all platforms.
 
     .. code-block:: ini
 
@@ -237,7 +237,7 @@ section of the command line docs.
 
           [tool.mypy]
           exclude = [
-              "^one\.py$",  # TOML's double-quoted strings require escaping backslashes
+              "^one\\.py$",  # TOML's double-quoted strings require escaping backslashes
               'two\.pyi$',  # but TOML's single-quoted strings do not
               '^three\.',
           ]

docs/source/literal_types.rst

@@ -292,8 +292,8 @@ using ``isinstance()``:
 This feature is sometimes called "sum types" or "discriminated union types"
 in other programming languages.
 
-Exhaustiveness checks
-*********************
+Exhaustiveness checking
+***********************
 
 You may want to check that some code covers all possible
 ``Literal`` or ``Enum`` cases. Example:
@@ -359,6 +359,35 @@ mypy will spot the error:
       # expected "NoReturn"
       assert_never(x)
 
+If runtime checking against unexpected values is not needed, you can
+leave out the ``assert_never`` call in the above example, and mypy
+will still generate an error about function ``validate`` returning
+without a value:
+
+.. code-block:: python
+
+  PossibleValues = Literal['one', 'two', 'three']
+
+  # Error: Missing return statement
+  def validate(x: PossibleValues) -> bool:
+      if x == 'one':
+          return True
+      elif x == 'two':
+          return False
+
+Exhaustiveness checking is also supported for match statements (Python 3.10 and later):
+
+.. code-block:: python
+
+  def validate(x: PossibleValues) -> bool:
+      match x:
+          case 'one':
+              return True
+          case 'two':
+              return False
+      assert_never(x)
+
+
 Limitations
 ***********
 
@@ -404,10 +433,10 @@ You can use enums to annotate types as you would expect:
   Movement(Direction.up, 5.0)  # ok
   Movement('up', 5.0)  # E: Argument 1 to "Movemement" has incompatible type "str"; expected "Direction"
 
-Exhaustive checks
-*****************
+Exhaustiveness checking
+***********************
 
-Similiar to ``Literal`` types ``Enum`` supports exhaustive checks.
+Similar to ``Literal`` types, ``Enum`` supports exhaustiveness checking.
 Let's start with a definition:
 
 .. code-block:: python
@@ -423,21 +452,22 @@ Let's start with a definition:
       up = 'up'
       down = 'down'
 
-Now, let's define an exhaustive check:
+Now, let's use an exhaustiveness check:
 
 .. code-block:: python
 
   def choose_direction(direction: Direction) -> None:
       if direction is Direction.up:
-          reveal_type(direction)  # N: Revealed type is "Literal[ex.Direction.up]"
+          reveal_type(direction)  # N: Revealed type is "Literal[Direction.up]"
           print('Going up!')
           return
       elif direction is Direction.down:
           print('Down')
           return
+      # This line is never reached
       assert_never(direction)
 
-And then test that it raises an error when some cases are not covered:
+If we forget to handle one of the cases, mypy will generate an error:
 
 .. code-block:: python
 
@@ -447,13 +477,13 @@ And then test that it raises an error when some cases are not covered:
           return
       assert_never(direction)  # E: Argument 1 to "assert_never" has incompatible type "Direction"; expected "NoReturn"
 
+Exhaustiveness checking is also supported for match statements (Python 3.10 and later).
+
 Extra Enum checks
 *****************
 
 Mypy also tries to support special features of ``Enum``
-the same way Python's runtime does.
-
-Extra checks:
+the same way Python's runtime does:
 
 - Any ``Enum`` class with values is implicitly :ref:`final <final_attrs>`.
   This is what happens in CPython:
@@ -467,7 +497,7 @@ Extra checks:
       ...
     TypeError: Other: cannot extend enumeration 'Some'
 
-  We do the same thing:
+  Mypy also catches this error:
 
   .. code-block:: python
 

docs/source/more_types.rst

@@ -874,68 +874,6 @@ value of type :py:class:`Coroutine[Any, Any, T] <typing.Coroutine>`, which is a
     :ref:`reveal_type() <reveal-type>` displays the inferred static type of
     an expression.
 
-If you want to use coroutines in Python 3.4, which does not support
-the ``async def`` syntax, you can instead use the :py:func:`@asyncio.coroutine <asyncio.coroutine>`
-decorator to convert a generator into a coroutine.
-
-Note that we set the ``YieldType`` of the generator to be ``Any`` in the
-following example. This is because the exact yield type is an implementation
-detail of the coroutine runner (e.g. the :py:mod:`asyncio` event loop) and your
-coroutine shouldn't have to know or care about what precisely that type is.
-
-.. code-block:: python
-
-   from typing import Any, Generator
-   import asyncio
-
-   @asyncio.coroutine
-   def countdown_2(tag: str, count: int) -> Generator[Any, None, str]:
-       while count > 0:
-           print('T-minus {} ({})'.format(count, tag))
-           yield from asyncio.sleep(0.1)
-           count -= 1
-       return "Blastoff!"
-
-   loop = asyncio.get_event_loop()
-   loop.run_until_complete(countdown_2("USS Enterprise", 5))
-   loop.close()
-
-As before, the result of calling a generator decorated with :py:func:`@asyncio.coroutine <asyncio.coroutine>`
-will be a value of type :py:class:`Awaitable[T] <typing.Awaitable>`.
-
-.. note::
-
-   At runtime, you are allowed to add the :py:func:`@asyncio.coroutine <asyncio.coroutine>` decorator to
-   both functions and generators. This is useful when you want to mark a
-   work-in-progress function as a coroutine, but have not yet added ``yield`` or
-   ``yield from`` statements:
-
-   .. code-block:: python
-
-      import asyncio
-
-      @asyncio.coroutine
-      def serialize(obj: object) -> str:
-          # todo: add yield/yield from to turn this into a generator
-          return "placeholder"
-
-   However, mypy currently does not support converting functions into
-   coroutines. Support for this feature will be added in a future version, but
-   for now, you can manually force the function to be a generator by doing
-   something like this:
-
-   .. code-block:: python
-
-      from typing import Generator
-      import asyncio
-
-      @asyncio.coroutine
-      def serialize(obj: object) -> Generator[None, None, str]:
-          # todo: add yield/yield from to turn this into a generator
-          if False:
-              yield
-          return "placeholder"
-
 You may also choose to create a subclass of :py:class:`~typing.Awaitable` instead:
 
 .. code-block:: python
@@ -995,11 +933,29 @@ To create an iterable coroutine, subclass :py:class:`~typing.AsyncIterator`:
    loop.run_until_complete(countdown_4("Serenity", 5))
    loop.close()
 
-For a more concrete example, the mypy repo has a toy webcrawler that
-demonstrates how to work with coroutines. One version
-`uses async/await <https://github.com/python/mypy/blob/master/test-data/samples/crawl2.py>`_
-and one
-`uses yield from <https://github.com/python/mypy/blob/master/test-data/samples/crawl.py>`_.
+If you use coroutines in legacy code that was originally written for
+Python 3.4, which did not support the ``async def`` syntax, you would
+instead use the :py:func:`@asyncio.coroutine <asyncio.coroutine>`
+decorator to convert a generator into a coroutine, and use a
+generator type as the return type:
+
+.. code-block:: python
+
+   from typing import Any, Generator
+   import asyncio
+
+   @asyncio.coroutine
+   def countdown_2(tag: str, count: int) -> Generator[Any, None, str]:
+       while count > 0:
+           print('T-minus {} ({})'.format(count, tag))
+           yield from asyncio.sleep(0.1)
+           count -= 1
+       return "Blastoff!"
+
+   loop = asyncio.get_event_loop()
+   loop.run_until_complete(countdown_2("USS Enterprise", 5))
+   loop.close()
+
 
 .. _typeddict: