CODEWITHSUNDEEP
pythonmethodspolymorphism
Paulo Freitas
Paulo Freitas

Reputation: 13649

Polymorphism in Python

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('...')

    def __read(self):
        raise NotImplementedError('Abstract method')

class FileA(File):
    def __read(self):
        pass

file = FileA('myfile.a')

# NotImplementedError: Abstract method

My question: what's wrong? How I can fix my code to FileA use FileA.__read() to read the file instead of File.__read()? :S

Thank you in advance.

Upvotes: 3

Views: 1085

Answers (2)

freegnu
freegnu

Reputation: 813

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.

Upvotes: 1

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798636

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.

Upvotes: 7

Related Questions