# Python Built-in Constants

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

## Boolean Constants

### `True`
The true value of the `bool` type. Assignments to `True` are illegal and raise a `SyntaxError`.

### `False`
The false value of the `bool` type. Assignments to `False` are illegal and raise a `SyntaxError`.

## Singleton Constants

### `None`
The sole value of the type `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`.

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

### `NotImplemented`
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.

## 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)` and `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` and `credits`
Objects that when printed or called, print the text of the 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 (if possible).

## Other Constants

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

[[programming/python/python]]