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ilevkivskyiilevkivskyi
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bpo-37045: PEP 591: Add final qualifiers to typing module (GH-13571)
The implementation is straightforward, it just mimics `ClassVar` (since the latter is also a name/access qualifier, not really a type). Also it is essentially copied from `typing_extensions`.
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‎Doc/library/typing.rst‎

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@@ -940,6 +940,31 @@ The module defines the following classes, functions and decorators:
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See :pep:`484` for details and comparison with other typing semantics.
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.. decorator:: final
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A decorator to indicate to type checkers that the decorated method
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cannot be overridden, and the decorated class cannot be subclassed.
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For example::
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class Base:
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@final
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def done(self) -> None:
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...
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class Sub(Base):
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def done(self) -> None: # Error reported by type checker
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...
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@final
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class Leaf:
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...
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class Other(Leaf): # Error reported by type checker
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...
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There is no runtime checking of these properties. See :pep:`591` for
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more details.
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.. versionadded:: 3.8
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.. decorator:: no_type_check
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Decorator to indicate that annotations are not type hints.
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.. versionadded:: 3.5.3
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.. data:: Final
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A special typing construct to indicate to type checkers that a name
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cannot be re-assigned or overridden in a subclass. For example::
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MAX_SIZE: Final = 9000
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MAX_SIZE += 1 # Error reported by type checker
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class Connection:
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TIMEOUT: Final[int] = 10
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class FastConnector(Connection):
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TIMEOUT = 1 # Error reported by type checker
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There is no runtime checking of these properties. See :pep:`591` for
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more details.
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.. versionadded:: 3.8
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.. data:: AnyStr
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``AnyStr`` is a type variable defined as

‎Lib/test/test_typing.py‎

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@@ -12,7 +12,7 @@
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from typing import Union, Optional
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from typing import Tuple, List, MutableMapping
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from typing import Callable
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from typing import Generic, ClassVar
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from typing import Generic, ClassVar, Final, final
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from typing import cast
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from typing import get_type_hints
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from typing import no_type_check, no_type_check_decorator
@@ -1438,6 +1438,53 @@ def test_no_isinstance(self):
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issubclass(int, ClassVar)
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class FinalTests(BaseTestCase):
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def test_basics(self):
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Final[int] # OK
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with self.assertRaises(TypeError):
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Final[1]
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with self.assertRaises(TypeError):
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Final[int, str]
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with self.assertRaises(TypeError):
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Final[int][str]
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with self.assertRaises(TypeError):
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Optional[Final[int]]
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def test_repr(self):
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self.assertEqual(repr(Final), 'typing.Final')
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cv = Final[int]
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self.assertEqual(repr(cv), 'typing.Final[int]')
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cv = Final[Employee]
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self.assertEqual(repr(cv), 'typing.Final[%s.Employee]' % __name__)
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def test_cannot_subclass(self):
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with self.assertRaises(TypeError):
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class C(type(Final)):
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pass
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with self.assertRaises(TypeError):
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class C(type(Final[int])):
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pass
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def test_cannot_init(self):
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with self.assertRaises(TypeError):
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Final()
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with self.assertRaises(TypeError):
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type(Final)()
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with self.assertRaises(TypeError):
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type(Final[Optional[int]])()
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def test_no_isinstance(self):
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with self.assertRaises(TypeError):
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isinstance(1, Final[int])
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with self.assertRaises(TypeError):
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issubclass(int, Final)
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def test_final_unmodified(self):
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def func(x): ...
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self.assertIs(func, final(func))
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class CastTests(BaseTestCase):
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def test_basics(self):

‎Lib/typing.py‎

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@@ -35,6 +35,7 @@
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'Any',
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'Callable',
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'ClassVar',
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'Final',
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'Generic',
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'Optional',
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'Tuple',
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# One-off things.
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'AnyStr',
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'cast',
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'final',
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'get_type_hints',
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'NewType',
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'no_type_check',
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"""
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invalid_generic_forms = (Generic, _Protocol)
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if is_argument:
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invalid_generic_forms = invalid_generic_forms + (ClassVar, )
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invalid_generic_forms = invalid_generic_forms + (ClassVar, Final)
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if arg is None:
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return type(None)
@@ -336,8 +338,8 @@ def __subclasscheck__(self, cls):
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@_tp_cache
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def __getitem__(self, parameters):
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if self._name == 'ClassVar':
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item = _type_check(parameters, 'ClassVar accepts only single type.')
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if self._name in ('ClassVar', 'Final'):
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item = _type_check(parameters, f'{self._name} accepts only single type.')
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return _GenericAlias(self, (item,))
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if self._name == 'Union':
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if parameters == ():
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be used with isinstance() or issubclass().
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""")
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Final = _SpecialForm('Final', doc=
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"""Special typing construct to indicate final names to type checkers.
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A final name cannot be re-assigned or overridden in a subclass.
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For example:
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MAX_SIZE: Final = 9000
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MAX_SIZE += 1 # Error reported by type checker
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class Connection:
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TIMEOUT: Final[int] = 10
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class FastConnector(Connection):
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TIMEOUT = 1 # Error reported by type checker
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There is no runtime checking of these properties.
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""")
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Union = _SpecialForm('Union', doc=
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"""Union type; Union[X, Y] means either X or Y.
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return _overload_dummy
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def final(f):
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"""A decorator to indicate final methods and final classes.
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Use this decorator to indicate to type checkers that the decorated
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method cannot be overridden, and decorated class cannot be subclassed.
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For example:
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class Base:
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@final
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def done(self) -> None:
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...
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class Sub(Base):
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def done(self) -> None: # Error reported by type checker
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...
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@final
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class Leaf:
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...
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class Other(Leaf): # Error reported by type checker
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...
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There is no runtime checking of these properties.
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"""
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return f
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class _ProtocolMeta(type):
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"""Internal metaclass for _Protocol.
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‎Misc/NEWS.d/next/Library/2019-05-25-18-36-50.bpo-37045.suHdVJ.rst‎

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PEP 591: Add ``Final`` qualifier and ``@final`` decorator to the ``typing`` module.

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