Polymorphism In Python

class File(object): def __init__(self, filename): if os.path.isfile(filename): self.filename = filename self.file = open(filename, 'rb')

Solution 1:

Prefixing an attribute with double underscores doesn't make the attribute private, it simply makes polymorphism impossible because the attribute name gets mangled with the current class name. Change it to a single underscore prefix instead.

Solution 2:

You can also leave the method undefined in the base class to achieve the same effect.

import os
classFile(object):
    def__init__(self, filename):
        if os.path.isfile(filename):
            self.filename = filename
            self.file = open(filename, 'rb')
            self._read()
        else:
            raise Exception('...')
classFileA(File):
    def_read(self):
        pass
file = FileA('myfile.a')

It is invaluable to the understanding of Python classes to have this understanding of class inheritance.