# 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.

  Note

  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.

  Note

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

  Changed in version 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.

Note

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.

help

  Object that when printed, prints the message “Type help() for interactive help, or help(object) for help about object.”, and when called, acts as described `elsewhere`.

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).