inheritance and default arguments

Andrew Koenig ark at research.att.com
Sun Aug 11 17:47:55 CEST 2002


I want a method in a class hierarchy to have a default argument.  For
example, I might want the method to deal with a designated substring
of a string that I pass as an argument, and I want the beginning and
end of the substring to be 0 and len(s) as defaults.

I can't do this:

        class Base(object):
                def f(self, s, begin=0, end=len(s)):
                        ...

        class Derived(object):
                def f(self, s, begin=0, end=len(s)):
                        ...

because the default argument is evaluated at the wrong time.
If I fix this problem in the obvious way:

        class Base(object):
                def f(self, s, begin=0, end=None):
                        if end == None:
                                end = len(s)
                        ...

        class Derived(base):
                def f(self, s, begin=0, end=None):
                        if end == None:
                                end = len(s)
                        ...

I must now repeat not only the default arguments, but also the
corresponding tests, in each derived class.

I can avoid that repetition by splitting the function into two:
One deals with the default-argument processing (and is inherited
by every derived class), the other does the actual work (and is
overridden as needed in the derived classes):

        class Base(object):
                def f(self, s, begin=0, end=None):
                        if end == None:
                                end = len(s)
                        return self.f_aux(s, begin, end)
                def f_aux(self, s, begin, end):
                        ...

        class Derived(object):
                def f_aux(self, s, begin, end):
                        ...

This isn't too bad, but I have the feeling that there may be a more
Pythonic way of doing the same thing.

Any suggestions?


-- 
Andrew Koenig, ark at research.att.com, http://www.research.att.com/info/ark



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