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May 2, 2023
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Improve assert_type phrasing #104081

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Improve assert_type phrasing
hauntsaninja May 2, 2023
9376fba
Update Lib/typing.py
hauntsaninja May 2, 2023
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7 changes: 4 additions & 3 deletions Doc/library/typing.rst
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Expand Up @@ -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::

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7 changes: 4 additions & 3 deletions Lib/typing.py
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Expand Up @@ -2308,15 +2308,16 @@ 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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