Private members in Python

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9.6. Private Variables

“Private” instance variables that
cannot be accessed except from inside
an object, don’t exist in Python.
However, there is a convention that is
followed by most Python code: a name
prefixed with an underscore (e.g.
_spam) should be treated as a non-public part of the API (whether it
is a function, a method or a data
member). It should be considered an
implementation detail and subject to
change without notice.

Since there is a valid use-case for
class-private members (namely to avoid
name clashes of names with names
defined by subclasses), there is
limited support for such a mechanism,
called name mangling. Any identifier
of the form __spam (at least two
leading underscores, at most one
trailing underscore) is textually
replaced with _classname__spam, where
classname is the current class name
with leading underscore(s) stripped.
This mangling is done without regard
to the syntactic position of the
identifier, as long as it occurs
within the definition of a class.

So, for example,

class Test:
    def __private_symbol(self):
        pass
    def normal_symbol(self):
        pass

print dir(Test)

will output:

['_Test__private_symbol', 
'__doc__', 
'__module__', 
'normal_symbol']

__private_symbol should be considered a private method, but it would still be accessible through _Test__private_symbol.

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