3. 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 toFalse
are illegal and raise aSyntaxError
.
-
True
¶ The true value of the
bool
type. Assignments toTrue
are illegal and raise aSyntaxError
.
-
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 toNone
are illegal and raise aSyntaxError
.
-
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; may be returned by the in-place binary special methods (e.g.__imul__()
,__iand__()
, etc.) for the same purpose. Its truth value is true.
Nota
When NotImplemented
is returned, the interpreter will then try the
reflected operation on the other type, or some other fallback, depending
on the operator. If all attempted operations return NotImplemented
, the
interpreter will raise an appropriate exception.
See Implementing the arithmetic operations for more details.
-
Ellipsis
¶ The same as
...
. Special value used mostly in conjunction with extended slicing syntax for user-defined container data types.
-
__debug__
¶ This constant is true if Python was not started with an
-O
option. See also theassert
statement.
Nota
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 «true» constants.
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.