PEP 285: Adding a bool type

[Laura Creighton]

+> 
> "Alex Martelli" <aleax at aleax.it> wrote in message
> news:gJDq8.55255$S52.1904212 at news2.tin.it...
> > John Roth wrote:
> >         ...
> > > Maybe my post in response didn't propagate? As I said in that
> > > post, the root of her problem is a poorly chosen name for the
> > > function. If she paid attention to constructing the name properly,
> >
> > She adopted the name *Guido* had given to the function right in
> > his pep (actually clarifying it a bit from 'isleap' in his PEP
> > to 'isLeapYear', more readable, in her post).
> 
> Sorry I'm late in responding to both posts - I had a problem
> with my newsgroup vendor and I'm just back on.
> 
> I thought the original example was very bad, but I didn't
> say so at the time because I hadn't figured out what I didn't
> like about it.
> 
> The original example was something like
> 
> number of days in year = 365 + isLeap(year).
> 
> To put it bluntly, that's a very confusing hack that depends
> on conceptual True being 1 and conceptual False being
> zero.
> 
> Again, to put it very bluntly, Laura swallowed a camel
> while straining at a gnat. She had a great deal to say about
> the difficulty of teaching that example, and I agree completely.
> It would be very difficult to teach, but the reason is that
> the name is simply wrong. The fact that Guido himself
> used that as an example doesn't make it right, either.
> 
> If she had corrected the name, then her entire basis for
> the objection would have evaporated.
> 
> John Roth
> >
> >
> > Alex
> >

Still haven't found the time to read my original post, notice that
I _did_ correct the name in the original post, and understand what
I was trying to say, I see.  I will correct your misconception
because you might confuse somebody else.

I am objecting to the fact that isleap, whatever the hell it is
called, returns a boolean.  I am rejecting the existence of
booleans in the language because it is too hard to teach how
to use them properly.  Most people never learn.  Most code that
uses booleans should not.

Proper use of booleans requires not only that you have a 
function that returns one of two values, but THAT YOU NEVER
EVER WANT THAT FUNCTION TO EVER BE MODIFIED TO RETURN ONE OF THREE,
(or more).  What I tell my students is that you really want 
a hole to open in the ceiling and a Warner Brothers safe to fall
on the head of somebody who attempts to extend, subclass, or 
in any other way change the function to return one of three values.

And, by God, if the function that returns _the number of leap days
in the year_ isn't safe, at 0 or 1, then damn near close to nothing
that my students are going to be tempted to use 'True' and 'False'
for is.  This example makes my course notes, under Why using booleans 
is bad, and you should never do it.  (unless you are a mathematician
who wants to do real 2-valued algebra, or somebody interfacing with
some other piece of software that needs those sort of bools, or 
some other, EXTREMELY RARE,  person.  Don't you guys interrupt here.
This is hard enough to teach as it is.)

It does not help that it appears to the outside that Guido just went
through all the python functions that returned 1 or 0 and changed
them to return True or False.  I know that Guido would not be
so stupid, and that there is a reason why he wants, for instance,
isinstance to return only 0 or 1 for evermore, but I can't see it.
I would like an isinstance that returned 0 for not an instance of
this class, 1 for a direct instance, and 2 for a subclass.  There
must be a lot of code out there I have never seen that relies on
this never happening, and now, alas, I have to find out why.

Because my students will ask, of course.  If True and False are
good enough for Guido, then it's good enough for them.  I would
love it if a huge comment was added to python files that now
return booleans saying why it was necessary to restrict the
return values to one of 2 forever more, so people are not confused
into thinking that returning booleans is a good idea in general,
but that may be too much to ask for.  I would add the comments
myself, but I am having problems seeing what the reasons are.
Maybe after I get some more sleep, it will come clearer to me.

I truly liked telling students, especailly ones that have never used
Python while we are using some other language,   'You don't need 
True and False. Look at Python.'  

Now I have to say, to the students who are using
Python: 'Python has True and False.  It is not for you little
         worms, and if I catch you using it I will make you do 
         the assignment over.'

I know in all the Python code I have ever written I have only 5 times
wanted a boolean True or False -- all times when I was talking to a
database that required it.  The rest of the time I wanted an integer,
0 or 1.  Even if I couldn't see a way that somebody was likely to
extend it to return 3, there most certainly was never ever going to be
any code that required that it only have one of 2 values, so now I
will have to change all my True and False there to be 'Yes' or 'No' or
something.  (grumble, now I wish I had lost the argument to the person
who wanted the constants we picked to be TRUE and FALSE).

Teaching people to not use booleans is hard.  They grab for them
whenever they have a function that returns one of two values.  It
was a lot easier to teach this concept when the fool things were not in
the language.

Note:  don't tell me that because python bools are just ints, it
       doesn't matter -- people who were using bools for
       DEMOCRAT and REPUBLICAN can now add 
       THE_DAMN_VOTE_WASNT_PUNCHED_THROUGH - (the integer 3)
       when to their horror they discover 2 values was not enough.
       I need to teach students to not use bools in languages which
       are not so forgiving.

Laura Creighton