"Private" (implementation) class in Python

In short:

1. You cannot enforce privacy. There are no private classes/methods/functions in Python. At least, not strict privacy as in other languages, such as Java.

2. You can only indicate/suggest privacy. This follows a convention. The Python convention for marking a class/function/method as private is to preface it with an _ (underscore). For example, def _myfunc() or class _MyClass:. You can also create pseudo-privacy by prefacing the method with two underscores (for example, __foo). You cannot access the method directly, but you can still call it through a special prefix using the classname (for example, _classname__foo). So the best you can do is indicate/suggest privacy, not enforce it.

Python is like Perl in this respect. To paraphrase a famous line about privacy from the Perl book, the philosophy is that you should stay out of the living room because you weren't invited, not because it is defended with a shotgun.

For more information:

• Private variables Python Documentation
• Why are Python’s ‘private’ methods not actually private? Stack Overflow question 70528

Define __all__, a list of names that you want to be exported (see documentation).

__all__ = ['public_class'] # don't add here the 'implementation_class'

A pattern that I sometimes use is this:

Define a class:

class x(object):
    def doThis(self):
        ...
    def doThat(self):
        ...

Create an instance of the class, overwriting the class name:

x = x()

Define symbols that expose the functionality:

doThis = x.doThis
doThat = x.doThat

Delete the instance itself:

del x

Now you have a module that only exposes your public functions.

Use a single underscore prefix:

class _Internal:
    ...

This is the official Python convention for 'internal' symbols; "from module import *" does not import underscore-prefixed objects.

Reference to the single underscore convention.