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[3.11] Improve assert_type phrasing (GH-104081) #104084

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Merged
AlexWaygood merged 1 commit into from
May 2, 2023
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[3.11] Improve assert_type phrasing (GH-104081) #104084

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Improve assert_type phrasing (GH-104081) 
I'd like to make the fact that this does nothing at runtime
really obvious, since I suspect this is unintuitive for users who are
unfamiliar with static type checking.

I thought of this because of
https://discuss.python.org/t/add-arg-check-type-to-types/26384
wherein I'm skeptical that the user really did want `assert_type`.
(cherry picked from commit 82ba6ce)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
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@hauntsaninja @miss-islington
hauntsaninja authored and miss-islington committed May 2, 2023
commit 74be0000ca8bd5a37dc6947693c2f1b7b06b3905
7 changes: 4 additions & 3 deletions Doc/library/typing.rst
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Expand Up @@ -2438,15 +2438,16 @@ Functions and decorators

Ask a static type checker to confirm that *val* has an inferred type of *typ*.

When the type checker encounters a call to ``assert_type()``, it
At runtime this does nothing: it returns the first argument unchanged with no
checks or side effects, no matter the actual type of the argument.

When a static type checker encounters a call to ``assert_type()``, it
emits an error if the value is not of the specified type::

def greet(name: str) -> None:
assert_type(name, str) # OK, inferred type of `name` is `str`
assert_type(name, int) # type checker error

At runtime this returns the first argument unchanged with no side effects.

This function is useful for ensuring the type checker's understanding of a
script is in line with the developer's intentions::

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7 changes: 4 additions & 3 deletions Lib/typing.py
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Expand Up @@ -2250,15 +2250,16 @@ def cast(typ, val):
def assert_type(val, typ, /):
"""Ask a static type checker to confirm that the value is of the given type.

When the type checker encounters a call to assert_type(), it
At runtime this does nothing: it returns the first argument unchanged with no
checks or side effects, no matter the actual type of the argument.

When a static type checker encounters a call to assert_type(), it
emits an error if the value is not of the specified type::

def greet(name: str) -> None:
assert_type(name, str) # ok
assert_type(name, int) # type checker error

At runtime this returns the first argument unchanged and otherwise
does nothing.
"""
return val

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