如何为 Python
__class_getitem__
类扩展 Generic
?我想向 __class_getitem__
添加参数,同时将一些参数向上传播到 Generic.__class_getitem__
。
请参阅下面的代码片段以获取示例用例(未运行):
from typing import ClassVar, Generic, TypeVar
T = TypeVar("T")
class Foo(Generic[T]):
cls_attr: ClassVar[int]
def __class_getitem__(cls, cls_attr: int, item):
cls.cls_attr = cls_attr
return super().__class_getitem__(item)
def __init__(self, arg: T):
pass
foo = Foo[1, bool](arg=True)
给我这个
TypeError
:
Traceback (most recent call last):
File "/path/to/file.py", line 17, in <module>
foo = Foo[1, bool](arg=True)
TypeError: Foo.__class_getitem__() missing 1 required positional argument: 'item'
正如 @juanpa.arrivilillaga 所建议的,这就是要走的路:
from typing import ClassVar, Generic, TypeVar
T = TypeVar("T")
class Foo(Generic[T]):
cls_attr: ClassVar[int]
def __class_getitem__(cls, item: tuple[int, T]):
cls.cls_attr = item[0]
return super().__class_getitem__(item[1])
def __init__(self, arg: T):
self.arg = arg
foo = Foo[1, bool](arg=True)
assert foo.cls_attr == 1
assert foo.arg
不幸的是,Python 类型检查工具似乎还不够先进,无法理解这种模式。例如,
mypy==0.971
(2022 年 9 月)尚不支持 __class_getitem__
,根据 https://github.com/python/mypy/issues/11501。
如果您希望有两个通用变量,那么您可以这样做:
from typing import Generic, TypeVar
T = TypeVar("T")
V = TypeVar("V")
class Foo(Generic[T, V]):
def __init__(self, arg: T):
pass
foo: Foo[int, bool] = Foo(arg=True)
我认为您可能正在寻找元类:
class CustomMeta(type):
def __getitem__(my_type_that_has_this_meta, my_values_passed_as_tuple_if_needed):
# do whatever
return WhateverTypeThatMatchesAnnotation()
class MyClass(metaclass=CustomMeta):
...
if __name__ == "__main__":
whatever = MyClass['hi', 'hello', 'hallo', 'bonjour']
assert isinstance(whatever, WhateverTypeThatMatchesAnnotation)
这使用了这样一个事实:就 Python 所知,元类是类的类型。
__class_getitem__
更漂亮,但在某些情况下,没有匹配的 typevar T
,我们只想传递非类型。在这些情况下,破解语言!
编辑:
我一直在研究它,并且由于它在 mypy 中也不起作用,因此您还可以使用
typing.TYPE_CHECKING
来绕过检查。例如,您可以创建以下代码
from typing import TYPE_CHECKING
class WhateverTypeThatMatchesAnnotation: ...
if not TYPE_CHECKING:
# python runs this
class CustomMeta(type):
def __getitem__(my_type_that_has_this_meta, my_values_passed_as_tuple_if_needed):
# do whatever
return WhateverTypeThatMatchesAnnotation()
class MyClass(metaclass=CustomMeta):
...
else:
# mypy runs this
class CustomMeta:
def __getitem__(self, item):
return WhateverTypeThatMatchesAnnotation()
MyClass = CustomMeta()
if __name__ == "__main__":
whatever = MyClass['hi', 'hello', 'hallo', 'bonjour']
assert isinstance(whatever, WhateverTypeThatMatchesAnnotation)