Python inherited class behaviour

Question

I have a question about python inherited class method, in the following code.

class B(object):
    def test(self):
        self.call()
    def call(self):
        print("Call from B")

if __name__ == "__main__":
    b = B()
    b.test()


from b import B

class C(B):
    def call(self):
        print("Call from C")

if __name__ == "__main__":
    c = C()
    c.test()

When I run this code, the result is

Call from C

The parent class method will call children's method. I want to know if it is an expected and stable behaviour? Because I also try the same logic in C++, it will print

Call from B

Answer 1

Yes, this is expected. c is an instance of C , but since C.test is not defined, c.test resolves to B.test . However, the corresponding call to self.call() invokes C.call because the runtime type of self is C , not B , and C.call is defined. Think of all Python methods as being virtual methods.

Answer 2

As a complement to chepner's answer, this C++ code exhibits the exact same behaviour as you have in Python:

#include <iostream>

class B {
public:
    void test() {
        call();
    }
    virtual void call() {
        std::cout << "Called from B" << std::endl;
    }
};

class C: public B {
public:
    void call() {
        std::cout << "Called from C" << std::endl;
    }
};

int main() {
   C c;
   c.test();        // will print Called from C
   return 0;
}

If you come from C++ think that all members are public, and all methods virtual.