This document is for an old version of Python that is no longer supported. You should upgrade and read the Python 当前稳定版本的文档.

# 4. 内置常量

有少数的常量存在于内置命名空间中。 它们是：

`False`

  The false value of the `bool` type.

  2.3 新版功能.

`True`

  The true value of the `bool` type.

  2.3 新版功能.

`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.

  在 2.4 版更改: 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__`

  如果 Python 没有以 `-O` 选项启动，则此常量为真值。 另请参见 `assert` 语句。

注解

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.

在 2.7 版更改: Assignments to `__debug__` as an attribute became illegal.

## 4.1. 由 `site` 模块添加的常量

`site` 模块（在启动期间自动导入，除非给出 `-S` 命令行选项）将几个常量添加到内置命名空间。 它们对交互式解释器 shell 很有用，并且不应在程序中使用。

`quit`([code=None])
`exit`([code=None])

  当打印此对象时，会打印出一条消息，例如“Use quit() or Ctrl-D (i.e. EOF) to exit”，当调用此对象时，将使用指定的退出代码来引发 `SystemExit`。

`copyright`
`credits`

  打印或调用的对象分别打印版权或作者的文本。

`license`

  当打印此对象时，会打印出一条消息“Type license() to see the full license text”，当调用此对象时，将以分页形式显示完整的许可证文本（每次显示一屏）。