How to do this in Python? - A "gotcha"

[Luis Zarrabeitia]

Quoting Jim Garrison <jhg at acm.org>:

> Jim Garrison wrote:
> > Luis Zarrabeitia wrote:
> >> On Tuesday 17 March 2009 06:04:36 pm Jim Garrison wrote:
> >> with open(filename, "rb") as f:
> >>     for buf in iter(lambda: f.read(1000),''):
> >>         do_something(buf)
> > 
> > This is the most pythonic solution yet.
> > 
> > Thanks to all the responders who took time to ponder this seemingly
> > trivial question.  I learned a lot about the Python mind-set.
> 
> I just tried the code as given above and it results in an infinite loop.
> 
> Since f.read() returns a byte string when in binary mode, the sentinel
> has to be b''.  Is there a value that will compare equal to both '' and b''?

Thank you for the correction.
It works in python2.5 (on my Debian, at least), but I can see why it doesn't in
python3.

> It's a shame the iter(o,sentinel) builtin does the
> comparison itself, instead of being defined as iter(callable,callable)
> where the second argument implements the termination test and returns a
> boolean.  

A shame indeed. The "takewhile" workaround is way too obfuscated, and to create
a new class only for this purpose is an overkill (and I'd also classify it as
obfuscated).

> This would seem to add much more generality... is
> it worthy of a PEP?

and, it wouldn't need to replace the current sentinel implementation... one
keyword argument, "predicate", would suffice.

+1.

Cheers,

-- 
Luis Zarrabeitia
Facultad de Matemática y Computación, UH
http://profesores.matcom.uh.cu/~kyrie


 Participe en Universidad 2010, del 8 al 12 de febrero de 2010
 La Habana, Cuba 
 http://www.universidad2010.cu