[Python-ideas] Calling a function of a list without accumulating results

[Terry Jones]

Hi Brett

| > The first is the same as one of the arguments for providing list
| > comprehensions and generator expressions - because it makes common
| > multi-line boilerplate much more concise.
| 
| OK, the question is how common is this.

I don't know. I use it maybe once every few weeks. There's a function to do
this in Common Lisp (mapc), not that that means anything much.

| But are you sure Python's grammar could support it?  Parentheses are
| needed for genexps in certain situations for disambiguation because of
| Python's LL(1) grammar.

I don't know the answer to this either. I imagine it's a matter of tacking
an optional "for ..." clause onto the end of an expression. The "for ..."
part can certainly be handled (is being handled already), so I think it
might not be too hard - supposing there is already a non-terminal for the
"for ..." clause.

| > The trivial case I posted isn't much of a win over the simple 2-line
| > alternative, but it's easy to go to further:
| >
| >     f(x, y) for x in myXlist for y in myYlist
| >
| > instead of
| >
| >     for x in myXlist:
| >         for y in myYlist:
| >             f(x, y)
| >
| > and of course there are many more examples.
| 
| Right, but the second one is so much easier to read and comprehend.

I tend to agree, but the language supports the more concise form for both
list comprehension and genexps, so there must be a fair number of people
who thought it was a win to allow the compact form.

| I think "force" is rather strong wording for "choice".

OK, how about "lack of choice"?  :-)

Seriously (to take an example from the Python pocket ref page 24), you do
have the choice to write

    a = [x for x in range(5) if x % 2 == 0]

instead of

    a = []
    for x in range(5):
        if x % 2 == 0:
            a.append(x)

but you don't have a (simple) choice if you don't want to accumulate
results.  I'm merely saying that I think it would be cleaner and more
consistent to allow

    print(x) for x in range(5) if x % 2 == 0

instead of having the non-choice but to write something like

    for x in range(5):
        if x % 2 == 0:
            print x

Yes, I (and thank you for it) could now use your suggested for _ in ...:
pass trick, but that's not really the whole point, to me. If the language
can be made simpler and more consistent, I think that's generally a good
thing.

I know, I don't know anything about the implementation. But this is an
ideas list.

| Heck, I would vote to ditch listcomps for ``list(genexp)`` had genexps
| come first and have the options trimmed down even more.

Me too. But even if that eventuated, I'd _still_ propose allowing the
unadorned genexp, for the case where you don't want the results.

| Basically, unless you can go through the stdlib and find all the
| instances of the pattern you want to prove it is common enough to
| warrant support it none of the core developers will probably go for
| this.

Understood.

Thanks,
Terry