Private functions and inheritance

Diez B. Roggisch deets at nospam.web.de
Mon Jul 16 16:54:48 CEST 2007


Maciej Bliziński wrote:

> Hello,
> 
> I've come across something that I don't quite understand about
> Python's inheritance. Consider the following code snippet:
> 
> class A(object):
>     def call_bar(self): return self.bar()
>     def call___bar(self): return self.__bar()
>     def __bar(self): return "A::__bar()"
>     def bar(self): return "A::bar()"
> 
> class B(A):
>     def __bar(self): return "B::__bar()"
>     def bar(self): return "B::bar()"
> 
> b = B()
> print "calling B::call_bar():", b.call_bar()
> print "calling B::call___bar():", b.call___bar()
> 
> The result is:
> 
> calling B::call_bar(): B::bar()
> calling B::call___bar(): A::__bar()
> 
> In the latter case, it calls the base class' implementation. It
> probably goes along with Python's spec, but I found it surprising. I
> don't want to expose the __bar() function outside, but on the other
> hand i want to defer its implementation to a subclass. It seems like I
> need to make it public, doesn't it?

Yep. Just use a single underscore, and that's it. The reason why this
doesn't work as expected is that the name-mangling introduced by the
leading double underscores is purely a lexical scope based mechanism.

Diez



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