Built-in Constants
A small number of constants live in the built-in namespace. They are:
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
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 ~types.NoneType type.
NotImplemented
A special value which should be returned by the binary special methods (e.g. ~object.__eq__, ~object.__lt__, ~object.__add__, ~object.__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. ~object.__imul__, ~object.__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
!NotImplementedthe 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!NotImplementedwill result in a misleading error message or the!NotImplementedvalue being returned to Python code.See
implementing-the-arithmetic-operationsfor examples.
[!CAUTION]
!NotImplementedand!NotImplementedErrorare not interchangeable. This constant should only be used as described above; seeNotImplementedErrorfor details on correct usage of the exception.
3.9 Evaluating !NotImplemented in a boolean context was deprecated.
3.14 Evaluating !NotImplemented in a boolean context now raises a TypeError. It previously evaluated to True and emitted a DeprecationWarning since Python 3.9.
single: ...; ellipsis literal
Ellipsis
The same as the ellipsis literal "...", an object frequently used to indicate that something is omitted. Assignment to Ellipsis is possible, but assignment to ... raises a SyntaxError. Ellipsis is the sole instance of the types.EllipsisType type.
__debug__
This constant is true if Python was not started with an -O option. See also the assert statement.
[!NOTE] The names
None,False,Trueand__debug__cannot be reassigned (assignments to them, even as an attribute name, raiseSyntaxError), so they can be considered "true" constants.
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 accessed directly in the interactive interpreter or called as functions, raise SystemExit with the specified exit code.
help
Object that when printed, prints the message "Type help() for interactive help, or help(object) for help about object.", and when accessed directly in the interactive interpreter, invokes the built-in help system (see help).
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).
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