Éric Araujo | 96deb75 | 2011-06-08 04:53:20 +0200 | [diff] [blame] | 1 | .. _built-in-consts: |
| 2 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 3 | Built-in Constants |
| 4 | ================== |
| 5 | |
| 6 | A small number of constants live in the built-in namespace. They are: |
| 7 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 8 | .. data:: False |
| 9 | |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 10 | The false value of the :class:`bool` type. Assignments to ``False`` |
Christian Heimes | 5b5e81c | 2007-12-31 16:14:33 +0000 | [diff] [blame] | 11 | are illegal and raise a :exc:`SyntaxError`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 12 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 13 | |
| 14 | .. data:: True |
| 15 | |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 16 | The true value of the :class:`bool` type. Assignments to ``True`` |
Christian Heimes | 5b5e81c | 2007-12-31 16:14:33 +0000 | [diff] [blame] | 17 | are illegal and raise a :exc:`SyntaxError`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 18 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 19 | |
| 20 | .. data:: None |
| 21 | |
R David Murray | e60e12b | 2012-07-22 20:43:13 -0400 | [diff] [blame] | 22 | The sole value of the type ``NoneType``. ``None`` is frequently used to |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 23 | represent the absence of a value, as when default arguments are not passed to a |
Christian Heimes | 5b5e81c | 2007-12-31 16:14:33 +0000 | [diff] [blame] | 24 | function. Assignments to ``None`` are illegal and raise a :exc:`SyntaxError`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 25 | |
| 26 | |
| 27 | .. data:: NotImplemented |
| 28 | |
Ethan Furman | b004943 | 2014-11-26 21:15:35 -0800 | [diff] [blame] | 29 | Special value which should be returned by the binary special methods |
| 30 | (e.g. :meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, :meth:`__rsub__`, |
| 31 | etc.) to indicate that the operation is not implemented with respect to |
| 32 | the other type; may be returned by the in-place binary special methods |
| 33 | (e.g. :meth:`__imul__`, :meth:`__iand__`, etc.) for the same purpose. |
| 34 | Its truth value is true. |
| 35 | |
| 36 | .. note:: |
| 37 | |
| 38 | When ``NotImplemented`` is returned, the interpreter will then try the |
| 39 | reflected operation on the other type, or some other fallback, depending |
| 40 | on the operator. If all attempted operations return ``NotImplemented``, the |
| 41 | interpreter will raise an appropriate exception. |
| 42 | |
| 43 | See |
| 44 | :ref:`implementing-the-arithmetic-operations` |
| 45 | for more details. |
| 46 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 47 | |
| 48 | |
| 49 | .. data:: Ellipsis |
| 50 | |
Georg Brandl | 9499bb7 | 2010-08-02 19:35:06 +0000 | [diff] [blame] | 51 | The same as ``...``. Special value used mostly in conjunction with extended |
| 52 | slicing syntax for user-defined container data types. |
Georg Brandl | 96593ed | 2007-09-07 14:15:41 +0000 | [diff] [blame] | 53 | |
| 54 | |
| 55 | .. data:: __debug__ |
| 56 | |
Christian Heimes | 5b5e81c | 2007-12-31 16:14:33 +0000 | [diff] [blame] | 57 | This constant is true if Python was not started with an :option:`-O` option. |
Christian Heimes | 5b5e81c | 2007-12-31 16:14:33 +0000 | [diff] [blame] | 58 | See also the :keyword:`assert` statement. |
Christian Heimes | 9bd667a | 2008-01-20 15:14:11 +0000 | [diff] [blame] | 59 | |
Georg Brandl | 0c7ade2 | 2010-08-02 19:39:17 +0000 | [diff] [blame] | 60 | |
Georg Brandl | c589a70 | 2010-08-02 19:36:36 +0000 | [diff] [blame] | 61 | .. note:: |
| 62 | |
| 63 | The names :data:`None`, :data:`False`, :data:`True` and :data:`__debug__` |
| 64 | cannot be reassigned (assignments to them, even as an attribute name, raise |
| 65 | :exc:`SyntaxError`), so they can be considered "true" constants. |
| 66 | |
Christian Heimes | 9bd667a | 2008-01-20 15:14:11 +0000 | [diff] [blame] | 67 | |
| 68 | Constants added by the :mod:`site` module |
| 69 | ----------------------------------------- |
| 70 | |
| 71 | The :mod:`site` module (which is imported automatically during startup, except |
| 72 | if the :option:`-S` command-line option is given) adds several constants to the |
| 73 | built-in namespace. They are useful for the interactive interpreter shell and |
| 74 | should not be used in programs. |
| 75 | |
Georg Brandl | c2a4f4f | 2009-04-10 09:03:43 +0000 | [diff] [blame] | 76 | .. data:: quit(code=None) |
| 77 | exit(code=None) |
Christian Heimes | 9bd667a | 2008-01-20 15:14:11 +0000 | [diff] [blame] | 78 | |
| 79 | Objects that when printed, print a message like "Use quit() or Ctrl-D |
| 80 | (i.e. EOF) to exit", and when called, raise :exc:`SystemExit` with the |
Benjamin Peterson | 8719ad5 | 2009-09-11 22:24:02 +0000 | [diff] [blame] | 81 | specified exit code. |
Christian Heimes | 9bd667a | 2008-01-20 15:14:11 +0000 | [diff] [blame] | 82 | |
| 83 | .. data:: copyright |
| 84 | license |
| 85 | credits |
| 86 | |
| 87 | Objects that when printed, print a message like "Type license() to see the |
| 88 | full license text", and when called, display the corresponding text in a |
| 89 | pager-like fashion (one screen at a time). |
| 90 | |