calling an overridden method from base class?

Question

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

dirty hack to get at parent `b.__class__.__mro__[1].hi(A())` — Jakob Bowyer, Jul 08 '11 at 06:50

score 9 · Answer 1 · answered Dec 02 '19 at 13:23

9

Note, this won't work for private methods in Python 3.6:

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 A.foo, not B.foo

I spent an hour to find out the reason for this

answered Dec 02 '19 at 13:23

Yakovlev Pavel

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Thanks to your comment I spent just 10 minutes while searching for the same! – IlyaP Jan 04 '21 at 12:58
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This is actually the *only purpose* for double-underscore name-mangling. – juanpa.arrivillaga Jul 28 '21 at 09:30
what *is* the reason for this? – Lachlan Mar 14 '22 at 15:20
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@IlyaP only 10 minutes? I think I've just spent an hour on it. Thanks Mr. Pavel! Stupid private Python methods that are not really private but also do not work as virtual protected method grumble mumble grumble I want my hour back!!! – user1593842 Jul 28 '23 at 15:42

score 8 · Accepted Answer · answered Jul 08 '11 at 06:46

8

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)

answered Jul 08 '11 at 06:46

Eli Bendersky

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oops! I made a blunder, I meant `b.bar()` both times in the original code: (and it still seemed obvious, since B inherits A, it has `bar()` too but I just cannot see it. Moreover since I am explicitly passing `self` it is clear it's B' object) – Vaibhav Bajpai Jul 08 '11 at 06:57
1

@Vaibhav: if this seems obvious, then good - what else? In C++, by the way, unless the methods are virtual it won't work this way. So it's not globally obvious. In Python, thinking in C++'s terms, all methods are virtual – Eli Bendersky Jul 08 '11 at 07:03
should I change my code to reflect the original question? (but I don't want to be rude since your answer corresponds to this one) – Vaibhav Bajpai Jul 08 '11 at 07:11
1

@Vaibhav: I guess you can change it, just note it explicitly with "Update". The important thing is that I hope you no longer are confused – Eli Bendersky Jul 08 '11 at 08:34

calling an overridden method from base class?

UPDATE

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