[Python-Dev] UserString

Guido van Rossum gvanrossum at gmail.com
Mon Feb 21 17:15:47 CET 2005

> > Anyway, can you explain why LBYL is bad?
> In the general case, it's bad because of a combination of issues.  It
> may violate "once, and only once!" -- the operations one needs to check
> may basicaly duplicate the operations one then wants to perform.  Apart
> from wasted effort, it may happen that the situation changes between
> the look and the leap (on an external file, or due perhaps to threading
> or other reentrancy).  It's often hard in the look to cover exactly the
> set of prereq's you need for the leap -- e.g. I've often seen code such
> as
>      if i < len(foo):
>          foo[i] = 24
> which breaks for i<-len(foo); the first time this happens the guard's
> changed to 0<=i<len(foo) which then stops the code from working
> w/negative index; finally it stabilizes to the correct check,
> -len(foo)<=i<len(foo), but even then it's just duplicating the same
> check that Python performs again when you then use foo[i]... just
> cluttering code.  The intermediate Pythonista's who's learned to code
> "try: foo[i]=24 // except IndexError: pass" is much better off than the
> one who's still striving to LBYL as he had (e.g.) when using C.
> Etc -- this is all very general and generic.

Right. There are plenty of examples where LBYL is better, e.g. because
there are too many different exceptions to catch, or they occur in too
many places. One of my favorites is creating a directory if it doesn't
already exist; I always use this LBYL-ish pattern:

 if not os.path.exists(dn):
    except os.error, err:
       ...log the error...

because the specific exception for "it already exists" is quite subtle
to pull out of the os.error structure.

Taken to th extreme, the "LBYL is bad" meme would be an argument
against my optional type checking proposal, which I doubt is what you

So, I'd like to take a much more balanced view on LBYL.

> I had convinced myself that strings were a special case worth singling
> out, via isinstance and basestring, just as (say) dictionaries are
> singled out quite differently by metods such as get... I may well have
> been too superficial in this conclusion.

I think there are lots of situations where the desire to special-case
strings is legitimate.

> >> Then you would be able to test whether something is sequence-like
> >> by the presence of __getitem__ or __iter__ methods, without
> >> getting tripped up by strings.
> >
> > There would be other ways to get out of this dilemma; we could
> > introduce a char type, for example. Also, strings might be
> > recognizable by other means, e.g. the presence of a lower() method or
> > some other characteristic method that doesn't apply to sequence in
> > general.
> Sure, there would many possibilities.
> > (To Alex: leaving transform() out of the string interface seems to me
> > the simplest solution.)
> I guess you mean translate.  Yes, that would probably be simplest.


BTW, there's *still* no sign from a PEP 246 rewrite. Maybe someone
could offer Clark a hand? (Last time I inquired he was recovering from
a week of illness.)

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

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