Question

Dive into Python –

Guido, the original author of Python, explains method overriding this way: “Derived classes may override methods of their base classes. Because methods have no special privileges when calling other methods of the same object, a method of a base class that calls another method defined in the same base class, may in fact end up calling a method of a derived class that overrides it. (For C++ programmers: all methods in Python are effectively virtual.)” If that doesn’t make sense to you (it confuses the hell out of me), feel free to ignore it. I just thought I’d pass it along.

I am trying to figure out an example for: a method of a base class that calls another method defined in the same base class, may in fact end up calling a method of a derived class that overrides it

class A:      
  def foo(self): print 'A.foo'      
  def bar(self): self.foo()                  

class B(A):      
  def foo(self): print 'B.foo'     

if __name__ == '__main__': 
  a = A()                
  a.bar()                   # echoes A.foo
  b = B()
  b.bar()                   # echoes B.foo

… but both of these seem kind of obvious.

am I missing something that was hinted out in the quote?

---

UPDATE

edited typo of calling a.foo() (instead of a.bar())and b.foo() (instead of b.bar()) in the original code

Answer 1

Yes, you’re missing this:

b.bar()   # echoes B.foo

B has no bar method of its own, just the one inherited from A. A‘s bar calls self.foo, but in an instance of B ends up calling B‘s foo, and not A‘s foo.

Let’s look at your quote again:

a method of a base class that calls
another method defined in the same
base class, may in fact end up calling
a method of a derived class that
overrides it

To translate:

bar (method of A, the base class)
calls self.foo, but may in fact end up
calling a method of the derived class
that overrides it (B.foo that
overrides A.foo)