| [391] | 1 | .. _built-in-consts:
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| 2 |
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| [2] | 3 | Built-in Constants
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| 4 | ==================
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| 5 |
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| 6 | A small number of constants live in the built-in namespace. They are:
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| 7 |
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| 8 | .. data:: False
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| 9 |
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| 10 | The false value of the :class:`bool` type.
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| 11 |
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| 12 | .. versionadded:: 2.3
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| 13 |
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| 14 |
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| 15 | .. data:: True
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| 16 |
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| 17 | The true value of the :class:`bool` type.
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| 18 |
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| 19 | .. versionadded:: 2.3
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| 20 |
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| 21 |
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| 22 | .. data:: None
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| 23 |
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| 24 | The sole value of :attr:`types.NoneType`. ``None`` is frequently used to
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| 25 | represent the absence of a value, as when default arguments are not passed to a
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| 26 | function.
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| 27 |
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| 28 | .. versionchanged:: 2.4
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| 29 | Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`.
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| 30 |
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| 31 |
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| 32 | .. data:: NotImplemented
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| 33 |
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| 34 | Special value which can be returned by the "rich comparison" special methods
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| 35 | (:meth:`__eq__`, :meth:`__lt__`, and friends), to indicate that the comparison
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| 36 | is not implemented with respect to the other type.
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| 37 |
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| 38 |
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| 39 | .. data:: Ellipsis
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| 40 |
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| 41 | Special value used in conjunction with extended slicing syntax.
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| 42 |
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| 43 |
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| 44 | .. data:: __debug__
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| 45 |
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| 46 | This constant is true if Python was not started with an :option:`-O` option.
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| 47 | See also the :keyword:`assert` statement.
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| 48 |
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| 49 |
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| [391] | 50 | .. note::
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| 51 |
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| 52 | The names :data:`None` and :data:`__debug__` cannot be reassigned
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| 53 | (assignments to them, even as an attribute name, raise :exc:`SyntaxError`),
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| 54 | so they can be considered "true" constants.
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| 55 |
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| 56 | .. versionchanged:: 2.7
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| 57 | Assignments to ``__debug__`` as an attribute became illegal.
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| 58 |
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| 59 |
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| [2] | 60 | Constants added by the :mod:`site` module
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| 61 | -----------------------------------------
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| 62 |
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| 63 | The :mod:`site` module (which is imported automatically during startup, except
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| 64 | if the :option:`-S` command-line option is given) adds several constants to the
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| 65 | built-in namespace. They are useful for the interactive interpreter shell and
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| 66 | should not be used in programs.
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| 67 |
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| 68 | .. data:: quit([code=None])
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| 69 | exit([code=None])
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| 70 |
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| 71 | Objects that when printed, print a message like "Use quit() or Ctrl-D
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| 72 | (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the
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| 73 | specified exit code.
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| 74 |
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| 75 | .. data:: copyright
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| 76 | license
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| 77 | credits
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| 78 |
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| 79 | Objects that when printed, print a message like "Type license() to see the
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| 80 | full license text", and when called, display the corresponding text in a
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| 81 | pager-like fashion (one screen at a time).
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