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Merged
hauntsaninja merged 6 commits into from
Sep 20, 2022
Merged

Update runtime_troubles docs #13680

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3e0efb5
Update runtime_troubles docs
hauntsaninja Sep 19, 2022
c4a893e
nit
hauntsaninja Sep 19, 2022
719964e
nit
hauntsaninja Sep 19, 2022
7b17a3f
another 311 ref
hauntsaninja Sep 19, 2022
40847ad
mention 3.12 in case we forget to update this
hauntsaninja Sep 19, 2022
512bc13
mention type comment syntax
hauntsaninja Sep 20, 2022
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53 changes: 38 additions & 15 deletions docs/source/runtime_troubles.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -8,8 +8,8 @@ version of Python considers legal code. This section describes these scenarios
and explains how to get your code running again. Generally speaking, we have
three tools at our disposal:

* For Python 3.7 through 3.9, use of ``from __future__ import annotations``
(:pep:`563`), made the default in Python 3.11 and later
* Use of ``from __future__ import annotations`` (:pep:`563`)
(this behaviour may eventually be made the default in a future Python version)
* Use of string literal types or type comments
* Use of ``typing.TYPE_CHECKING``

Expand All @@ -18,20 +18,42 @@ problems you may encounter.

.. _string-literal-types:

String literal types
--------------------
String literal types and type comments
--------------------------------------

Mypy allows you to add type annotations using ``# type:`` type comments.
For example:

.. code-block:: python

a = 1 # type: int

def f(x): # type: (int) -> int

@JukkaL JukkaL Sep 20, 2022

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By the way, are multi-line function type comments documented somewhere, i.e. comments like these:

def f(x,  # type: int
      y,  # type: str
):
    # type: (...) -> None
    ...

@hauntsaninja hauntsaninja Sep 20, 2022 •
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Not any more, I think they were only documented in the Python 2 cheatsheet (RIP)

return x + 1

# Alternative type comment syntax for functions with many arguments
def send_email(
address, # type: Union[str, List[str]]
sender, # type: str
cc, # type: Optional[List[str]]
subject='',
body=None # type: List[str]
):
# type: (...) -> bool

Type comments can't cause runtime errors because comments are not evaluated by
Python. In a similar way, using string literal types sidesteps the problem of
Python.

In a similar way, using string literal types sidesteps the problem of
annotations that would cause runtime errors.

Any type can be entered as a string literal, and you can combine
string-literal types with non-string-literal types freely:

.. code-block:: python

def f(a: list['A']) -> None: ... # OK
def g(n: 'int') -> None: ... # OK, though not useful
def f(a: list['A']) -> None: ... # OK, prevents NameError since A is defined later
def g(n: 'int') -> None: ... # Also OK, though not useful

class A: pass

Expand All @@ -47,9 +69,10 @@ Future annotations import (PEP 563)
-----------------------------------

Many of the issues described here are caused by Python trying to evaluate
annotations. From Python 3.11 on, Python will no longer attempt to evaluate
function and variable annotations. This behaviour is made available in Python
3.7 and later through the use of ``from __future__ import annotations``.
annotations. Future Python versions (potentially Python 3.12) will by default no
longer attempt to evaluate function and variable annotations. This behaviour is
made available in Python 3.7 and later through the use of
``from __future__ import annotations``.

This can be thought of as automatic string literal-ification of all function and
variable annotations. Note that function and variable annotations are still
Expand All @@ -74,7 +97,7 @@ required to be valid Python syntax. For more details, see :pep:`563`.
class B: ...
class C: ...

.. note::
.. warning::

Some libraries may have use cases for dynamic evaluation of annotations, for
instance, through use of ``typing.get_type_hints`` or ``eval``. If your
Expand Down Expand Up @@ -273,8 +296,8 @@ the built-in collections or those from :py:mod:`collections.abc`:
y: dict[int, str]
z: Sequence[str] = x

There is limited support for using this syntax in Python 3.7 and later as well.
If you use ``from __future__ import annotations``, mypy will understand this
There is limited support for using this syntax in Python 3.7 and later as well:
if you use ``from __future__ import annotations``, mypy will understand this
syntax in annotations. However, since this will not be supported by the Python
interpreter at runtime, make sure you're aware of the caveats mentioned in the
notes at :ref:`future annotations import<future-annotations>`.
Expand All @@ -285,8 +308,8 @@ Using X | Y syntax for Unions
Starting with Python 3.10 (:pep:`604`), you can spell union types as ``x: int |
str``, instead of ``x: typing.Union[int, str]``.

There is limited support for using this syntax in Python 3.7 and later as well.
If you use ``from __future__ import annotations``, mypy will understand this
There is limited support for using this syntax in Python 3.7 and later as well:
if you use ``from __future__ import annotations``, mypy will understand this
syntax in annotations, string literal types, type comments and stub files.
However, since this will not be supported by the Python interpreter at runtime
(if evaluated, ``int | str`` will raise ``TypeError: unsupported operand type(s)
Expand Down