True inconsistency in Python

KefX keflimarcusx at aol.comNOSPAM
Mon Nov 17 13:29:28 CET 2003


>Explicit comparison with the true constant (or false constant)
>necessarily degenerates into complete uselessness.

(For those just joining us, this statement was made about all languages, not
just Python.)

I'm not so sure about that. In most languages (including Python), 'false' is
guaranteed to be only one value (usually zero), therefore comparing against it
poses no problem. I assume you're saying that doing so isn't problematic in any
way, it's just pointless.

Well, actually, some coding standards suggest you always compare against false.
This is so the code is slightly more clear, and "!= false" avoids the pitfall
that "== true" has. The problem of course is that some unwitting maintenance
programmer may decide that "== true" os more clear than "!= false", not
realizing it is wrong...which is why all maintenance programmers should read
programming standards. :) We can't program in baby C just for them, after all,
but we shouldn't make things unnecessarily difficult for them either, but I
think we have bigger things to worry about if a maintenance programmer fails to
understand such a simple concept after being told about it.

Now of course any idiot C (or C++, or Java) programmer knows that these two are
equivalent:

if(blah != false)
  do_blah();

if(blah)
  do_blah();

But if whatever goes in "blah" is really long (and often it is), you very
quickly see what is being compared: it's a straight boolean comparison, whereas
with the second you have to look at the whole thing, find no comparison
operator, and go, "oh, there's no explicit comparison so it's obviously an
implicit straight boolean comparison". This is especially valuable in really
long 'if' statements, such as one might want for an inverse parser (don't worry
if you don't know what one is). The idea is if you compare against 'false' all
the time, you won't forget it when it would actually give the code more
clarity.

One standard which suggests this is the one Steve Maguire proposes in the book
Writing Solid Code. This is probably the best book on C coding guidelines that
I've ever read, despite it being written by a Microsoft programmer. ;) Although
much of it is specific to C, a lot of the concepts apply to other languages,
including Python.

However, comparing against a boolean is discouraged in Python (but your
argument wasn't about Python specifically), so I wouldn't do it in Python: when
in Rome, do as the Romans do. Although, as a last thought, it should be
relatively easy to make it so that True returns '1' in any comparison except
against 0. A class would be able to do this easily by overriding __cmp__:

def true_class(bool):
  def __cmp__(self, other):
    if other:
      return 1
    else:
      return 0

True = true_class()

(of course this class definition would be tweaked to make it a singleton)
I'm assuming there's a good reason for not doing it this way (or in an
equivalent fashion), though...what else might we add to this discussion?

- Kef





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