A small number of constants live in the built-in namespace. They are:
The false value of the bool type. Assignments to False are illegal and raise a SyntaxError.
The true value of the bool type. Assignments to True are illegal and raise a SyntaxError.
An object 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. None is the sole instance of the NoneType type.
A 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; may be returned by the in-place binary special methods (e.g. __imul__(), __iand__(), etc.) for the same purpose. It should not be evaluated in a boolean context. NotImplemented is the sole instance of the types.NotImplementedType type.
Note
When a binary (or in-place) method returns NotImplemented the interpreter will try the reflected operation on the other type (or some other fallback, depending on the operator). If all attempts return NotImplemented, the interpreter will raise an appropriate exception. Incorrectly returning NotImplemented will result in a misleading error message or the NotImplemented value being returned to Python code.
See Implementing the arithmetic operations for examples.
Caution
NotImplemented and NotImplementedError are not interchangeable. This constant should only be used as described above; see NotImplementedError for details on correct usage of the exception.
Changed in version 3.9: Evaluating NotImplemented in a boolean context is deprecated. While it currently evaluates as true, it will emit a DeprecationWarning. It will raise a TypeError in a future version of Python.
The same as the ellipsis literal â...â. Special value used mostly in conjunction with extended slicing syntax for user-defined container data types. Ellipsis is the sole instance of the types.EllipsisType type.
This constant is true if Python was not started with an -O option. See also the assert statement.
Note
The names None, False, True and __debug__ cannot be reassigned (assignments to them, even as an attribute name, raise SyntaxError), so they can be considered âtrueâ constants.
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.
Objects that when printed, print a message like âUse quit() or Ctrl-D (i.e. EOF) to exitâ, and when called, raise SystemExit with the specified exit code.
Object that when printed, prints the message âType help() for interactive help, or help(object) for help about object.â, and when called, acts as described elsewhere.
Objects that when printed or called, print the text of copyright or credits, respectively.
Object that when printed, prints the message âType license() to see the full license textâ, and when called, displays the full license text in a pager-like fashion (one screen at a time).
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