[SOLVED] Python overriding inherited private methods

Snark1994 · 09-24-2014, 10:27 AM

Hi,

I was playing around with python and noticed something:

Code:

#!/usr/bin/env python3

class PrivateParent:
    def do_something(self):
        self.__print_message()

    def __print_message(self):
        print("In Parent!")

class PrivateChild(PrivateParent):
    def __print_message(self):
        print("In Child!")

foo = PrivateChild()
foo.do_something()

will print out "In Parent!", while

Code:

#!/usr/bin/env python3

class PublicParent:
    def do_something(self):
        self.print_message()

    def print_message(self):
        print("In Parent!")

class PublicChild(PublicParent):
    def print_message(self):
        print("In Child!")

foo = PublicChild()
foo.do_something()

will print out "In Child!".

I want the second behaviour, but it doesn't really make sense to expose the method in question publicly. Is there any way around this?

Additionally, does anyone have a good explanation for why this is?

Thanks,

mina86 · 09-24-2014, 10:41 AM

__foo identifiers are mangled to _ClassName__foo form so __print_message is _PrivateParent__print_message in PrivateParent class and _PrivateChild__print_message in PrivateChild class.

Code:

class PrivateParent:
    def do_something(self):
        self.__print_message()

    def __print_message(self):
        print("In Parent!")

class PrivateChild(PrivateParent):
    def _PrivateParent__print_message(self):
        print("In Child!")

foo = PrivateChild()
foo.do_something()

You should, of course, try to avoid doing this. Trying to access private members of other classes may indicate fault in your design. If the data/method is meant to be accessed from children, it should be protected (or public). If it's not meant to be accessed, child should not touch it.

Snark1994 · 09-24-2014, 10:47 AM

Ah, I knew they were mangled but couldn't remember exactly how.

Hm, I guess you're right that if the child is overriding, then the method "should" be public, hadn't really thought of it that way.

Thanks!

dugan · 09-24-2014, 11:42 AM

Quote:

Originally Posted by mina86

If the data/method is meant to be accessed from children, it should be protected (or public).

Python has no concept of "protected", but starting identifier names with single underscores is a common way to signify it.

mina86 · 09-24-2014, 01:46 PM

Quote:

Originally Posted by dugan

Python has no concept of "protected", but starting identifier names with single underscores is a common way to signify it.

Sure, if you want to be anal about it, Python has no concept of “private” either, but starting identifier name with two underscores mangles it.