python: callable函数

assert callable(0) == False
assert callable("Hello Python") == False

def add(a, b):
    return a + b
assert callable(add) == True

class A:
    def method(self):
        return 0
assert callable(A) == True
a = A()
assert callable(a) == False

class B:
    def __call__(self):
        return 0
assert callable(B) == True
b = B()
assert callable(b) == True  # 这边为什么是 True 我也没搞懂，等后面再回过头来研究一下