[英]Python using child class's class attribute as default parameter without overriding method
[英]Python overriding type hint on a method's return in child class, without redefining method signature
我有一个基础 class 在方法返回时带有float
类型提示。
在子 class 中,在不重新定义签名的情况下,我可以以某种方式将方法返回的类型提示更新为int
吗?
示例代码
#!/usr/bin/env python3.6
class SomeClass:
"""This class's some_method will return float."""
RET_TYPE = float
def some_method(self, some_input: str) -> float:
return self.RET_TYPE(some_input)
class SomeChildClass(SomeClass):
"""This class's some_method will return int."""
RET_TYPE = int
if __name__ == "__main__":
ret: int = SomeChildClass().some_method("42"). #
ret2: float = SomeChildClass().some_method("42")
我的 IDE 抱怨类型不匹配:
发生这种情况是因为我的 IDE 仍在使用SomeClass.some_method
的类型提示。
研究
我认为解决方案可能是使用 generics,但我不确定是否有更简单的方法。
建议可能使用实例变量注释,但我不确定如何为返回类型执行此操作。
以下代码在 PyCharm 上运行良好。 我添加了complex
的案例以使其更清晰。
我基本上将该方法提取到通用 class 中,然后将其用作每个子类的 mixin。 请格外小心使用,因为它似乎相当不标准。
from typing import ClassVar, Generic, TypeVar, Callable
S = TypeVar('S', bound=complex)
class SomeMethodImplementor(Generic[S]):
RET_TYPE: ClassVar[Callable]
def some_method(self, some_input: str) -> S:
return self.__class__.RET_TYPE(some_input)
class SomeClass(SomeMethodImplementor[complex]):
RET_TYPE = complex
class SomeChildClass(SomeClass, SomeMethodImplementor[float]):
RET_TYPE = float
class OtherChildClass(SomeChildClass, SomeMethodImplementor[int]):
RET_TYPE = int
if __name__ == "__main__":
ret: complex = SomeClass().some_method("42")
ret2: float = SomeChildClass().some_method("42")
ret3: int = OtherChildClass().some_method("42")
print(ret, type(ret), ret2, type(ret2), ret3, type(ret3))
例如,如果您将ret2: float
更改为ret2: int
,它将正确显示类型错误。
可悲的是, mypy在这种情况下确实显示错误(版本 0.770),
otherhint.py:20: error: Incompatible types in assignment (expression has type "Type[float]", base class "SomeClass" defined the type as "Type[complex]")
otherhint.py:24: error: Incompatible types in assignment (expression has type "Type[int]", base class "SomeClass" defined the type as "Type[complex]")
otherhint.py:29: error: Incompatible types in assignment (expression has type "complex", variable has type "float")
otherhint.py:30: error: Incompatible types in assignment (expression has type "complex", variable has type "int")
第一个错误可以通过编写来“修复”
RET_TYPE: ClassVar[Callable] = int
对于每个子类。 现在,错误减少到
otherhint.py:29: error: Incompatible types in assignment (expression has type "complex", variable has type "float")
otherhint.py:30: error: Incompatible types in assignment (expression has type "complex", variable has type "int")
这与我们想要的恰恰相反,但如果你只关心 PyCharm,那也没关系。
你可以使用类似的东西:
from typing import TypeVar, Generic
T = TypeVar('T', float, int) # types you support
class SomeClass(Generic[T]):
"""This class's some_method will return float."""
RET_TYPE = float
def some_method(self, some_input: str) -> T:
return self.RET_TYPE(some_input)
class SomeChildClass(SomeClass[int]):
"""This class's some_method will return int."""
RET_TYPE = int
if __name__ == "__main__":
ret: int = SomeChildClass().some_method("42")
ret2: float = SomeChildClass().some_method("42")
但是有一个问题。 那我不知道怎么解决。 对于 SomeChildClass 方法 some_method IDE 将显示通用提示。 至少 pycharm(我想你是这个)不会将其显示为错误。
好的,所以我可以尝试并结合@AntonPomieshcheko 和@KevinLanguasco 的答案来提出一个解决方案,其中:
mypy
报告这正是我想要的行为。 非常感谢大家:)
#!/usr/bin/env python3
from typing import TypeVar, Generic, ClassVar, Callable
T = TypeVar("T", float, int) # types supported
class SomeBaseClass(Generic[T]):
"""This base class's some_method will return a supported type."""
RET_TYPE: ClassVar[Callable]
def some_method(self, some_input: str) -> T:
return self.RET_TYPE(some_input)
class SomeChildClass1(SomeBaseClass[float]):
"""This child class's some_method will return a float."""
RET_TYPE = float
class SomeChildClass2(SomeBaseClass[int]):
"""This child class's some_method will return an int."""
RET_TYPE = int
class SomeChildClass3(SomeBaseClass[complex]):
"""This child class's some_method will return a complex."""
RET_TYPE = complex
if __name__ == "__main__":
some_class_1_ret: float = SomeChildClass1().some_method("42")
some_class_2_ret: int = SomeChildClass2().some_method("42")
# PyCharm can infer this return is a complex. However, running mypy on
# this will report (this is desirable to me):
# error: Value of type variable "T" of "SomeBaseClass" cannot be "complex"
some_class_3_ret = SomeChildClass3().some_method("42")
print(
f"some_class_1_ret = {some_class_1_ret} of type {type(some_class_1_ret)}\n"
f"some_class_2_ret = {some_class_2_ret} of type {type(some_class_2_ret)}\n"
f"some_class_3_ret = {some_class_3_ret} of type {type(some_class_3_ret)}\n"
)
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