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

2002年4月03日 13:56:52 -0500

> It's the *speaking*. When i want to say true or false, then there's
> the dilemma.
>> I know, your answer is "you should always just say True or False",
> and Mark McEahern said the same thing. But this cannot be so in
> practice: *everything* already returns 0 or 1. (It may be possible to
> get around this if we commit to changing the entire standard library
> before releasing a version of Python with True and False, but alas,
> this is not the only issue... read on.)
>> As long as True and False are somewhere represented as 0 and 1,
> the values 0 and 1 will never lose legitimacy as booleans. This
> business with str() and/or repr() producing "0" or "1" for backwards
> compatibility prevents us from considering 0 and 1 relegated to a
> truly non-boolean status.

But that's a variant of the PEP that no-one except Marc-Andre has
spoken in favor of. The PEP proposes str(True) == "True".
> Consider this:
>> >>> a = [0, False, 1, True]
>> >>> print a
> [0, 0, 1, 1]
>> >>> for x in a: print x
> 0
> 0
> 1
> 1
>> Good heavens!

It will print
 0
 False
 1
 True
> What about this:
>> >>> d = {}
> >>> d[0] = 'zero'
> >>> d[False] = 'false'
> >>> len(d)
> 1 or 2?

What about this:
 >>> d = {}
 >>> d[0] = 'int'
 >>> d[0.0] = 'float'
 >>> d[0j] = 'complex'
 >>> print len(d)
 1 or 3?
False and True are numbers, and they are equal (==) to 0 and 1;
everything else follows from there.
> Basically, the concept of having a permanently schizophrenic type
> in the language scares me. The above shows, i believe, that a
> reasonable implementation must print True as True and False as False,
> and never mention 1 or 0.

And this is what the PEP proposes (despite its brief mention of an
alternative).
> Moreover, as soon as you start sorting a
> bag of objects, or keying dictionaries on objects, you are forced to
> run into the distinction between 0 and False, and between 1 and True.

No you're not. d[0] and d[False] retrieve the same value, as do
d[0L], d[0.0], and d[0j].
> I'm not against the idea of booleans, of course -- but i do think
> that halfway booleans are worse than what we have now. And getting
> to real booleans [*] involves real pain; it's just a question of
> whether that pain is worth it. Even if we get all the way there --
> as in we manage to convert enough code and convince everyone to use
> the new style -- i will never ever want "and" and "or" to return
> booleans (i just hope that doesn't confuse anyone).
>>> -- ?!ng
>> [*] By real booleans, i mean the following. (Booleans would have
> to behave like this for me to consider them "good enough" to
> be better than what we have now.)
>> >>> False, repr(False), str(False)
> (False, 'False', 'False')
> >>> True, repr(False), str(False)
> (True, 'False', 'False')
> >>> False + True
> TypeError...

That's just one textbook idea of what a Boolean "should" be.
> >>> False == None
> 0
> >>> False == 0
> 0
> >>> True == 1
> 0
> >>> {0: 0, False: False, 1: 1, True: True}
> {0: 0, False: False, 1: 1, True: True}
>> ... and probably
>> >>> None < False < True < 0
> True
>> (Hee hee -- i suppose the fact that "boolean" starts with a "b"
> gets us this for free. But i wonder how many people are going
> to be puzzled by True < 0?)

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