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

Gerald S. Williams gsw@agere.com
Wed, 3 Apr 2002 13:08:13 -0500

Mark McEahern wrote:

If, as Fredrik says, there's already one obvious way to return True (and that's the int 1), then we already have the problem that people will be inclined to compare something to this canonical truth in conditional statements.

Actually, he wrote "preferred", not "obvious".

I.e.: def bool(x): return not not x will return 0 or 1.

but (), [], "", and None also evaluate as false in a logical context, while most other things evaluate as true.

Having canonical True and False values would lead newcomers to compare to them. This would work, for example:

DONE = y and (x > y) ... if DONE is False: ...

On the other hand, this wouldn't (unless y is a boolean):

DONE = (x > y) and y ... if DONE is False: ...

Of course, it works either way if you replace "DONE is False" with "not DONE". Or if you replace "y" with "bool(y)".

Since there aren't canonical True/False values, there is currently an obvious solution: use "not DONE". But "native" boolean support will lead to mistakes like this unless you start taking steps to introduce type-safety with operations involving boolean values. This is definitely going farther than was intended by the PEP.

-Jerry

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