[Python-Dev] For review: PEP 285: Adding a bool type

[Guido van Rossum]

I'll let this short and sweet PEP speak for itself.

http://python.sourceforge.net/peps/pep-0285.html

PEP: 285
Title: Adding a bool type
Version: $Revision: 1.1 $
Last-Modified: $Date: 2002/03/08 15:38:37 $
Author: guido@python.org (Guido van Rossum)
Status: Draft
Type: Standards Track
Created: 8-Mar-2002
Python-Version: 2.3
Post-History: 8-Mar-2002 (python-dev)


Abstract

    This PEP proposes the introduction of a new built-in type, bool,
    with two constants, False and True.  The bool type would be a
    straightforward subtype (in C) of the int type, and the values
    False and True would behave like 0 and 1 in most respects (e.g.
    False==0 and True==1 would be true) except repr() and str().  All
    built-in operations that conceptually return a Boolean result will
    be changed to return False or True instead of 0 or 1; for example,
    comparisons and the "not" operator.


Rationale

    Most languages eventually grow a Boolean type; even C99 has one.
    It's useful to be able to tell from a function result that the
    outcome has Boolean semantics.


Specification

    The following Python code specifies most of the properties of the
    new type:

        class bool(int):

            def __new__(cls, val=0, _create=0):
                if _create:
                    # This is nor part of the spec,
                    # just a hack to bootstrap False and True
                    return int.__new__(cls, not not val)
                elif val:
                    return True
                else:
                    return False

            def __repr__(self):
                if self:
                    return "True"
                else:
                    return "False"

            __str__ = __repr__

            def __and__(self, other):
                if isinstance(other, bool):
                    return bool(int(self) & int(other))
                else:
                    return NotImplemented

            __rand__ = __and__

            def __or__(self, other):
                if isinstance(other, bool):
                    return bool(int(self) | int(other))
                else:
                    return NotImplemented

            __ror__ = __or__

            def __xor__(self, other):
                if isinstance(other, bool):
                    return bool(int(self) ^ int(other))
                else:
                    return NotImplemented

            __rxor__ = __xor__


        False = bool(0, _create=1)
        True = bool(1, _create=1)


Issues

    Because the repr() or str() of a bool value is different from an
    int value, some code (e.g. doctest-based unit tests) may fail.
    How much of a backwards compatibility problem this will be, I
    don't know.  If we find this is a real problem, we could add a
    command-line option to make False and True aliases for 0 and 1 and
    bool an alias for int.

                                   
Copyright

    This document has been placed in the public domain.



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--Guido van Rossum (home page: http://www.python.org/~guido/)