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2.3.2 Reserved classes of identifiers

Certain classes of identifiers (besides keywords) have special meanings. These classes are identified by the patterns of leading and trailing underscore characters:

‘_*’

Not imported by ‘from module import *’. The special identifier ‘_’ is used in the interactive interpreter to store the result of the last evaluation; it is stored in the ‘__builtin__’ module. When not in interactive mode, ‘_’ has no special meaning and is not defined. See section 6.12, "The import statement." Note: The name ‘_’ is often used in conjunction with internationalization; refer to the documentation for the ‘gettext’ module for more information on this convention.

‘__*__’

System-defined names. These names are defined by the interpreter and its implementation (including the standard library); applications should not expect to define additional names using this convention. The set of names of this class defined by Python may be extended in future versions. See section 3.4, "Special method names."

‘__*’

Class-private names. Names in this category, when used within the context of a class definition, are re-written to use a mangled form to help avoid name clashes between "private" attributes of base and derived classes. See section 5.2.1, "Identifiers (Names)."