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Improve assert_type phrasing (#104081)

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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`.
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Shantanu 2023-05-01 23:05:25 -07:00 • committed by GitHub
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2 changed files with 8 additions and 6 deletions
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7
Doc/library/typing.rst
View file

@ -2484,15 +2484,16 @@ 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::

7
Lib/typing.py
View file

@ -2319,15 +2319,16 @@ 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