问题
The following code does not work as expected. Apparently, I cannot use the classes' own type inside class definition:
class Foo:
def __init__(self, key :str) -> None:
self.key = key
def __eq__(self, other :Foo) -> bool:
return self.key == other.key
print('should be true: ', Foo('abc') == Foo('abc'))
print('should be false: ', Foo('abc') == Foo('def'))
The result of running it is:
Traceback (most recent call last):
File "class_own_type.py", line 1, in <module>
class Foo:
File "class_own_type.py", line 5, in Foo
def __eq__(self, other :Foo) -> bool:
NameError: name 'Foo' is not defined
Also, checking the code with mypy returns:
class_own_type.py:5: error: Argument 1 of "__eq__" incompatible with supertype "object"
How can I correct this code to be valid, both for Python and for mypy?
回答1:
The name Foo isn't bound yet, because the class itself has not yet been defined at the time that you try to use the name (remember: function arguments are evaluated at function definition time, not at function call time).
You can use string literals, to delay evaluation of the type:
class Foo:
def __init__(self, key :str) -> None:
self.key = key
def __eq__(self, other: 'Foo') -> bool:
return self.key == other.key
print('should be true: ', Foo('abc') == Foo('abc'))
print('should be false: ', Foo('abc') == Foo('def'))
来源：https://stackoverflow.com/questions/42845972/typed-python-using-the-classes-own-type-inside-class-definition