Polymorphism in Python

Question

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


        

Answer 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.

Answer 2

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

import os
class File(object):
    def __init__(self, filename):
        if os.path.isfile(filename):
            self.filename = filename
            self.file = open(filename, 'rb')
            self._read()
        else:
            raise Exception('...')
class FileA(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.