Guido van Rossum wrote:
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.
+1.
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__
I don't like this: it will break too much code since there are already two singletons Py_True and Py_False available in Python and these return 1 / 0 resp.
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)
Please adjust Py_True and Py_False (at C level) to be identical to these constants.
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.
Please, no ! Ideal would be to make bool a subtype of int and True and False two singletons created from 1 and 0 having the bool type: that's simple and backwards compatible. One idea for a possible addition: Add boolean support to the array module so that it becomes possible to create arrays of bits.
Copyright
This document has been placed in the public domain.
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--Guido van Rossum (home page: http://www.python.org/~guido/)
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