EDIT: Justus provided in the comments a great link where Jim Fulton argues that __ should be marked deprecated, folllowed by Neal Norwiz and Tim Peters answers. This helps a lot understanding what __ should be used for: http://mail.python.org/pipermail/python-dev/2005-December/058555.html

I am writing on Python OOP best practices and I was wondering what are the best ways to name attributes in classes. My main concern is about the distinction between private and protected attributes.
- private attributes are attributes that cannot be seen or used outside of the class, even buy subclasses. The Python parser calls the name mangling algorithm when it finds them to prevent name collision; - protected attributes are attributes that can be used and seen in subclasses and should not be used outside. Nothing is done on them and they can be used like public attributes.

When should we use them ?

If you read PEP8, it's clearly said that name mangling (using a '__' prefix) is the best way to protect an attribute from beeing accessed or overriden. So it should be used for all class internals that is not intended to be overriden.

But in the biggest open source code bases like Zope or Plone, '__' usage is very uncommon. The simple '_' prefix is often used instead, to mark attributes that are private to the class or to the module. So there are no real distinction between private and protected attributes. It seems that the 'private' concept is not even used, and people often cut their class code in two parts: public and protected.

In other languages (like Delphi) that define protected and private levels though, protected attributes are not used a lot, and people tend to cut their code in private and public parts and make the protected layer as slim as possible.

Practical rules

Based on these remarks, here's a tentative of '__' and '_' prefixes best usages in Python, for the use cases I know :
- use __ with property. since properties cannot use overriden methods and are tied to the class, the methods used with it should always be private; - use __ for methods that works with private attributes. If your methods works for private attributes, make them private too; - use _ on methods when they are clearly intended to be overriden; - use __ for all module functions and variables that are private. A protected level is not needed since a module cannot be overriden.

Following these rules would probably make 90% of class attributes private instead of protected, and change all base code conventions. So I am wondering: am I a bit unpythonic if I try to follow this standard in attribute naming ? My guess is that most base code are not clean enough in that matter. For instance, many of them use both new-style and old-style classes under Python 2.5, which lead to a MRO algorithm that differs depending on the classes !

I would love to hear how you people deal with these conventions.