# Built-in Constants

A small number of constants live in the built-in namespace. They are:

`False`

  The false value of the `bool` type. Assignments to `False` are illegal and raise a `SyntaxError`.

`True`

  The true value of the `bool` type. Assignments to `True` are illegal and raise a `SyntaxError`.

`None`

  An object frequently used to represent the absence of a value, as when default arguments are not passed to a function. Assignments to `None` are illegal and raise a `SyntaxError`. `None` is the sole instance of the `NoneType` type.

`NotImplemented`

  A special value which should be returned by the binary special methods (e.g. `__eq__()`, `__lt__()`, `__add__()`, `__rsub__()`, etc.) to indicate that the operation is not implemented with respect to the other type; may be returned by the in-place binary special methods (e.g. `__imul__()`, `__iand__()`, etc.) for the same purpose. It should not be evaluated in a boolean context. `NotImplemented` is the sole instance of the `types.NotImplementedType` type.

  Nota

  When a binary (or in-place) method returns `NotImplemented` the interpreter will try the reflected operation on the other type (or some other fallback, depending on the operator). If all attempts return `NotImplemented`, the interpreter will raise an appropriate exception. Incorrectly returning `NotImplemented` will result in a misleading error message or the `NotImplemented` value being returned to Python code.

  See Implementing the arithmetic operations for examples.

  Nota

  `NotImplementedError` and `NotImplemented` are not interchangeable, even though they have similar names and purposes. See `NotImplementedError` for details on when to use it.

  Cambiato nella versione 3.9: Evaluating `NotImplemented` in a boolean context is deprecated. While it currently evaluates as true, it will emit a `DeprecationWarning`. It will raise a `TypeError` in a future version of Python.

`Ellipsis`

  The same as the ellipsis literal «`...`». Special value used mostly in conjunction with extended slicing syntax for user-defined container data types. `Ellipsis` is the sole instance of the `types.EllipsisType` type.

`__debug__`

  This constant is true if Python was not started with an `-O` option. See also the `assert` statement.

Nota

The names `None`, `False`, `True` and `__debug__` cannot be reassigned (assignments to them, even as an attribute name, raise `SyntaxError`), so they can be considered «true» constants.

## 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.

`copyright`
`credits`

  Objects that when printed or called, print the text of copyright or credits, respectively.

`license`

  Object that when printed, prints the message «Type license() to see the full license text», and when called, displays the full license text in a pager-like fashion (one screen at a time).