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Add assert_type #1103

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Merged
merged 4 commits into from
Mar 22, 2022
Merged

Add assert_type #1103

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9ffa1e1
Add assert_type
JelleZijlstra Mar 17, 2022
e6084a0
living in the past
JelleZijlstra Mar 17, 2022
4dbae2a
Merge branch 'master' into asserttype
JelleZijlstra Mar 22, 2022
c0ac1b8
Update typing_extensions/src/test_typing_extensions.py
JelleZijlstra Mar 22, 2022
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1 change: 1 addition & 0 deletions typing_extensions/CHANGELOG
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
# Unreleased

- Add `typing.assert_type`. Backport from bpo-46480.
- Drop support for Python 3.6. Original patch by Adam Turner (@AA-Turner).

# Release 4.1.1 (February 13, 2022)
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1 change: 1 addition & 0 deletions typing_extensions/README.rst
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Expand Up @@ -46,6 +46,7 @@ This module currently contains the following:
- In ``typing`` since Python 3.11

- ``assert_never``
- ``assert_type``
- ``LiteralString`` (see PEP 675)
- ``Never``
- ``NotRequired`` (see PEP 655)
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19 changes: 17 additions & 2 deletions typing_extensions/src/test_typing_extensions.py
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Expand Up @@ -12,7 +12,7 @@
from unittest import TestCase, main, skipUnless, skipIf
from test import ann_module, ann_module2, ann_module3
import typing
from typing import TypeVar, Optional, Union, Any
from typing import TypeVar, Optional, Union, Any, AnyStr
from typing import T, KT, VT # Not in __all__.
from typing import Tuple, List, Dict, Iterable, Iterator, Callable
from typing import Generic, NamedTuple
Expand All @@ -23,7 +23,7 @@
from typing_extensions import Awaitable, AsyncIterator, AsyncContextManager, Required, NotRequired
from typing_extensions import Protocol, runtime, runtime_checkable, Annotated, overload, final, is_typeddict
from typing_extensions import TypeVarTuple, Unpack, dataclass_transform, reveal_type, Never, assert_never, LiteralString
from typing_extensions import get_type_hints, get_origin, get_args
from typing_extensions import assert_type, get_type_hints, get_origin, get_args

# Flags used to mark tests that only apply after a specific
# version of the typing module.
Expand Down Expand Up @@ -425,6 +425,21 @@ def blah():
blah()


class AssertTypeTests(BaseTestCase):

def test_basics(self):
arg = 42
self.assertIs(assert_type(arg, int), arg)
self.assertIs(assert_type(arg, Union[str, float]), arg)
self.assertIs(assert_type(arg, AnyStr), arg)
self.assertIs(assert_type(arg, None), arg)

def test_errors(self):
# Bogus calls are not expected to fail.
arg = 42
self.assertIs(assert_type(arg, 42), arg)
self.assertIs(assert_type(arg, 'hello'), arg)


T_a = TypeVar('T_a')

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21 changes: 21 additions & 0 deletions typing_extensions/src/typing_extensions.py
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Expand Up @@ -717,6 +717,27 @@ class Film(TypedDict):
"""
return isinstance(tp, tuple(_TYPEDDICT_TYPES))


if hasattr(typing, "assert_type"):
assert_type = typing.assert_type

else:
def assert_type(__val, __typ):
"""Assert (to the type checker) that the value is of the given type.
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@hauntsaninja hauntsaninja Mar 17, 2022 •
edited
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More a critique of the original PR, but not sure that "Assert (to the type checker) that the value is of the given type." is the least confusing phrasing.

You're not telling the type checker a fact, you're asking it to confirm one, so maybe "Ask a static type checker to confirm that the value is of the given type".

Or at least, we should do more to distinguish it from assert isinstance(val, typ) (whose behaviour is what I would describe as "asserting something to the type checker")

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Could you bring this up on cpython instead? The docstring here is just a copy of the CPython one. We can sync it back later if we want.

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Sure. Should I open a PR directly against the original BPO or would you prefer I file its own BPO

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We can keep the same one


When the 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


if hasattr(typing, "Required"):
get_type_hints = typing.get_type_hints
else:
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