A small number of constants live in the built-in namespace. They are:
The sole value oftypes.NoneType.None is frequently used torepresent the absence of a value, as when default arguments are not passed to afunction.
Changed in version 2.4:Assignments toNone are illegal and raise aSyntaxError.
Special value which can be returned by the “rich comparison” special methods(__eq__(),__lt__(), and friends), to indicate that the comparisonis not implemented with respect to the other type.
Special value used in conjunction with extended slicing syntax.
This constant is true if Python was not started with an-O option.See also theassert statement.
Note
The namesNone and__debug__ cannot be reassigned(assignments to them, even as an attribute name, raiseSyntaxError),so they can be considered “true” constants.
Changed in version 2.7:Assignments to__debug__ as an attribute became illegal.
Thesite module (which is imported automatically during startup, exceptif the-S command-line option is given) adds several constants to thebuilt-in namespace. They are useful for the interactive interpreter shell andshould not be used in programs.
Objects that when printed, print a message like “Use quit() or Ctrl-D(i.e. EOF) to exit”, and when called, raiseSystemExit with thespecified exit code.
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