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# 3. Built-in Constants

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

Note

`None`, `False`, `True` and `__debug__` cannot be reassigned (assignments to them raise `SyntaxError`), so they can be considered “true” constants.

`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`
  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. 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`

  The same as `...`. Special value used mostly in conjunction with extended slicing syntax for user-defined container data types, as in

      .. XXX Someone who understands extended slicing should fill in here.

`__debug__`
  This constant is true if Python was not started with an -O option. Assignments to `__debug__` are illegal and raise a `SyntaxError`. See also the `assert` statement.

## 3.1. 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`
`license`
`credits`
  Objects that when printed, print a message like “Type license() to see the full license text”, and when called, display the corresponding text in a pager-like fashion (one screen at a time).

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