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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 3.3, ``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).''