Private Variables and Methods in Python

Question

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The meaning of a single- and a double-underscore before an object name in Python

Which should I use _foo (an underscore) or __bar (double underscore) for private members and methods in Python?

Answer 1

Please note that there is no such thing as "private method" in Python. Double underscore is just name mangling:

>>> class A(object):
...     def __foo(self):
...         pass
... 
>>> a = A()
>>> A.__dict__.keys()
['__dict__', '_A__foo', '__module__', '__weakref__', '__doc__']
>>> a._A__foo()

So therefore __ prefix is useful when you need the mangling to occur, for example to not clash with names up or below inheritance chain. For other uses, single underscore would be better, IMHO.

EDIT, regarding confusion on __, PEP-8 is quite clear on that:

If your class is intended to be subclassed, and you have attributes that you do not want subclasses to use, consider naming them with double leading underscores and no trailing underscores. This invokes Python's name mangling algorithm, where the name of the class is mangled into the attribute name. This helps avoid attribute name collisions should subclasses inadvertently contain attributes with the same name.

Note 3: Not everyone likes name mangling. Try to balance the need to avoid accidental name clashes with potential use by advanced callers.

So if you don't expect subclass to accidentally re-define own method with same name, don't use it.