This document is for an old version of Python that is no longer supported. You should install the python3 and python3-doc packages and read the Python documentation for the Python3 version packaged in this release.

# 4. Built-in Constants

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

False

  The false value of the `bool` type.

  New in version 2.3.

True

  The true value of the `bool` type.

  New in version 2.3.

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.

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).