[mkesper]

"Private" fields and methods should use one underscore. Two underscores are for name mangling issues and __method__ is for "magic" methods.

[zardeh]

I'd say that private methods are double underscored, and protected methods are single underscored, since the goal of the __ is to prevent child classes from being able to use the parent implementation via self.__meth.

[xapata]

Not prevented from being able to use, but from accidentally using or overriding the parent's. The child can still use the mangled name.

[guyzmo]

Well, private fields are all members starting with an underscore and that are not publicised in the documentation.

/That/ includes double underscores mangled members and and "magic" methods.