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# 4. 內建常數

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

`False`

  The false value of the `bool` type.

  2.3 版新加入.

`True`

  The true value of the `bool` type.

  2.3 版新加入.

`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.

  2.4 版更變: 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.

備註

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.

2.7 版更變: 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).