Python：可以在内置类型上调用 __subclasses__()，而不是在自定义对象上

Question

class Test: pass
print(Test.__subclasses__())

returns:

AttributeError: class Test has no attribute '__subclasses__'

And

print(int.__subclasses__())

returns:

[<type 'bool'>]

Why can't I call subclasses () on my custom object?

This build in methods aren't reserver for custom types, are they?

Each class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. Example:

Answer 1

You will need to do this:-

class Foo(object):
    pass

# This works perfectly fine now.
print(Foo.__subclasses__())

The class above is a "new-style" class because it inherits from the object class. New-style classes provide a lot of extra framework that "old-style" classes do not have. One particular attribute of a new-style class is to be able to determine the subclasses of the class with the subclasses method.