[Python-Dev] Re: PEP 285: Adding a bool type (original) (raw)

Guido van Rossum guido@python.org
Wed, 03 Apr 2002 12:12:36 -0500

The addition of another type really does change the set of preferred truth values from {0, 1} to {0, False, 1, True}. The typical practicing Python programmer who wants to say "true" would have to decide -- for each instance -- whether to say "1" or "True".

No. If they want to use the recommended values they should always use False or True. 0 and 1 are relegated to the domain of non-representative values of the set of false and true objects, just as None and "true" (the string literal).

And the programmer cannot happily ignore the issue and stick to using 0 and 1, because if comparisons start to return True and False, dealing with a mix of four truth values is unavoidable. This PEP makes me uneasy because it causes me to see lots of extra casts in Python's future (not to mention confused beginners).

You misunderstand it. Strings, lists, numbers and so on are still acceptable as truth values, and when you want to know whether x is true or false, you still have to say "if x:" -- never "if x == True:".

In addition, for backward compatibility 0 == False and 1 == True, so mixing them shouldn't be a problem if you know your values are chosen from the set {0, False, 1, True}.

If you find yourself comparing an unknown value x to a known truth value to determine whether x is true or false, stop right there: you've already got a problem. If you don't have a problem because it works in practice (meaning x is known to be 0 or 1) the backwards compatibility of the PEP kicks in and saves your butt. This won't be deprecated: the PEP promises that False==0 and True==1 will always hold. (There's a question for reviewers about this, but consider it decided -- bools will always mix freely with ints.)

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