diff --git a/docs/source/literal_types.rst b/docs/source/literal_types.rst index da62b31526c96..d5271b256bb21 100644 --- a/docs/source/literal_types.rst +++ b/docs/source/literal_types.rst @@ -1,7 +1,10 @@ +Literal types and Enums +======================= + .. _literal_types: Literal types -============= +------------- Literal types let you indicate that an expression is equal to some specific primitive value. For example, if we annotate a variable with type ``Literal["foo"]``, @@ -369,3 +372,118 @@ whatever type the parameter has. For example, ``Literal[3]`` is treated as a subtype of ``int`` and so will inherit all of ``int``'s methods directly. This means that ``Literal[3].__add__`` accepts the same arguments and has the same return type as ``int.__add__``. + + +Enums +----- + +Mypy has special support for :py:class:`enum.Enum` and its subclasses: +:py:class:`enum.IntEnum`, :py:class:`enum.Flag`, and :py:class:`enum.IntFlag`. + +.. code-block:: python + + from enum import Enum + + class Direction(Enum): + up = 'up' + down = 'down' + + reveal_type(Direction.up) # Revealed type is "Literal[Direction.up]?" + reveal_type(Direction.down) # Revealed type is "Literal[Direction.down]?" + +You can use enums to annotate types as you would expect: + +.. code-block:: python + + class Movement: + def __init__(self, direction: Direction, speed: float) -> None: + self.direction = direction + self.speed = speed + + Movement(Direction.up, 5.0) # ok + Movement('up', 5.0) # E: Argument 1 to "Movemement" has incompatible type "str"; expected "Direction" + +Exhaustive checks +***************** + +Similiar to ``Literal`` types ``Enum`` supports exhaustive checks. +Let's start with a definition: + +.. code-block:: python + + from enum import Enum + from typing import NoReturn + + def assert_never(value: NoReturn) -> NoReturn: + # This also works in runtime as well: + assert False, 'This code should never be reached, got: {0}'.format(value) + + class Direction(Enum): + up = 'up' + down = 'down' + +Now, let's define an exhaustive 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]" + print('Going up!') + return + elif direction is Direction.down: + print('Down') + return + assert_never(direction) + +And then test that it raises an error when some cases are not covered: + +.. code-block:: python + + def choose_direction(direction: Direction) -> None: + if direction == Direction.up: + print('Going up!') + return + assert_never(direction) # E: Argument 1 to "assert_never" has incompatible type "Direction"; expected "NoReturn" + +Extra Enum checks +***************** + +Mypy also tries to support special features of ``Enum`` +the same way Python's runtime does. + +Extra checks: + +- Any ``Enum`` class with values is implicitly :ref:`final `. + This is what happens in CPython: + + .. code-block:: python + + >>> class AllDirection(Direction): + ... left = 'left' + ... right = 'right' + Traceback (most recent call last): + ... + TypeError: Other: cannot extend enumeration 'Some' + + We do the same thing: + + .. code-block:: python + + class AllDirection(Direction): # E: Cannot inherit from final class "Some" + left = 'left' + right = 'right' + +- All ``Enum`` fields are implictly ``final`` as well. + + .. code-block:: python + + Direction.up = '^' # E: Cannot assign to final attribute "up" + +- All field names are checked to be unique. + + .. code-block:: python + + class Some(Enum): + x = 1 + x = 2 # E: Attempted to reuse member name "x" in Enum definition "Some"