Python Cookbook

Delegating Automatically as an Alternative to Inheritance

Credit: Alex Martelli

Problem

You’d like to inherit from a built-in type, but you are using Python 2.1 (or earlier), or need a semantic detail of classic classes that would be lost by inheriting from a built-in type in Python 2.2.

Solution

With Python 2.2, we can inherit directly from a built-in type. For example, we can subclass file with our own new-style class and override some methods:

class UppercaseFile(file):
    def write(self, astring):
        return file.write(self, astring.upper(  ))
    def writelines(self, strings):
        return file.writelines(self, map(string.upper,strings))
upperOpen = UppercaseFile

To open such a file, we can call upperOpen just like a function, with the same arguments as the built-in open function. Because we don’t override _ _init_ _, we inherit file’s arguments, which are the same as open’s.

If we are using Python 2.1 or earlier, or if we need a classic class for whatever purpose, we can use automatic delegation:

class UppercaseFile:
    # Initialization needs to be explicit
    def _ _init_ _(self, file):
        # NOT self.file=file, to avoid triggering _ _setattr_ _ 
        self._ _dict_ _['file'] = file # Overrides aren't very different from the inheritance case: def write(self, astring): return self.file.write(astring.upper( )) def writelines(self, strings): return self.file.writelines(map(string.upper,strings)) # Automatic delegation is a simple and short boilerplate: def _ _getattr_ _(self, attr): return getattr(self.file, attr) ...