[Python-Dev] List comprehensions

M.-A. Lemburg mal@lemburg.com
Thu, 13 Jul 2000 21:51:59 +0200

Paul Prescod wrote:
> "M.-A. Lemburg" wrote:
> >
> > ...
> >
> > Is it really necessary to have all the "code" inside the square
> > brackets ?
> That's what a list comprehension is! Check the defacto PEP:
> http://www.cosc.canterbury.ac.nz/~greg/python/listcomp/
> I take it you are against list comprehensions.

If it means writing code in square brackets: yes -- 
I don't see any real benefit other than confusing the reader...
it doesn't even gain any performance (I would have thought that
constant ranges like [0:10] would generate [0,1,2,3,4,5,6,7,8,9]
and make this a constant, but that doesn't seem to be the case).
> > I always thought of list comprehension as a fast way to
> > just throw together a list, not as a way to "program" the
> > list contents. IMHO, that should be done using explicit
> > statements in the way Skip describes above.
> >
> > I would be completely satisfied with e.g. [0..10], [0..10:2]
> > and [10..0].
> Those are not list comprehensions but "range literals."

Oh, so I won't get these with list comprehension ? Hmm, then
I'm posting on the wrong thread, I guess ;-)

> Actually, the
> syntax we are using is [0:10], [0:10:2], [10:0]. I prefer dotdot, but
> Guido chose the colon a decade ago.

Say, wouldn't it make more sense to
have these notations generate tuples (which are immutable and
could thus be stored as constants) rather than lists ?!

for x in (0:10):
   print x

Hmm, looks strange, perhaps not such a good notation....

for x in (0..10):
   print x

or even:

for x in 0..10:
   print x

Oh, now this looks much better :-) 

The for-loop opcode could even make some use of this by not generating
a tuple at all (it could use a special mutable counter like the
one available in my speedup patch for 1.5:


[see the ceval part of the patch -- it uses the counter object
the patch also contains which provides a mutable integer
implementation for just this purpose]

The patch also includes some other performance enhancements
such as a small dictionary enhancement and many other things
which have already made it into 2.0)

Marc-Andre Lemburg
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