4. Built-in Constants — Python 2.7.10 documentation
A small number of constants live in the built-in namespace. They are:
- None¶
The sole value of types.NoneType. None is frequently used to represent the absence of a value, as when default arguments are not passed to a function.
Changed in version 2.4: Assignments to None are illegal and raise a SyntaxError.
- NotImplemented¶
Special value which can be returned by the “rich comparison” special methods (__eq__(), __lt__(), and friends), to indicate that the comparison is not implemented with respect to the other type.
- Ellipsis¶
Special value used in conjunction with extended slicing syntax.
- __debug__¶
This constant is true if Python was not started with an -O option. See also the assert statement.
Note
The names None and __debug__ cannot be reassigned (assignments to them, even as an attribute name, raise SyntaxError), so they can be considered “true” constants.
Changed in version 2.7: Assignments to __debug__ as an attribute became illegal.
4.1. Constants added by the site module¶
The site module (which is imported automatically during startup, except if the -S command-line option is given) adds several constants to the built-in namespace. They are useful for the interactive interpreter shell and should not be used in programs.
- quit([code=None])¶
- exit([code=None])¶
Objects that when printed, print a message like “Use quit() or Ctrl-D (i.e. EOF) to exit”, and when called, raise SystemExit with the specified exit code.