[Python-ideas] if in for-loop statement

[Henk-Jaap Wagenaar]

One does not seem to be able to do this in a generator expression:

foo = (x for x in [1, 2] if True else [1, 2, 3])

gives a syntax error, however, adding parenthesis 'solves' this:

foo = (x for x in [1, 2] if True else [1, 2, 3])

In the for-loop version, either works. Though I guess this would be even
more frowned upon, one can even do:

for x in [], print("Here be dragons"):
    pass

Whether it can be implemented in an LL(1) parser, my gut says it could, but
this does complicate matters if one were to continue supporting it and
create a whirl of confusion.

If anything, this shows that there could have been scope for more
consistency between the for-statement and generator expression by enforcing
parenthesis in the for-loop as well but I think the grammar as it is will
be here to stay, unless there is going to be a Python 4 like there is
Python 3...

I think to be honest this is a pretty big nail in the coffin.

H-J



On 23 February 2017 at 14:51, Jelle Zijlstra <jelle.zijlstra at gmail.com>
wrote:

> 2017-02-23 5:37 GMT-08:00 Henk-Jaap Wagenaar <wagenaarhenkjaap at gmail.com>:
> >
> > Hi all,
> >
> > Often I have typed something like
> >
> >     for x in range(100) if is_prime(x):
> >         # do things with x
> >
> > to find that this does not work, instead resorting to:
> >
> >     for x in range(100):
> >         if is_prime(x):
> >             # do things with x
> >
> > or
> >
> >     for x in range(100):
> >         if not is_prime(x):
> >             continue
> >         # do things with x
> >
> > Other solutions to another case of this 'problem' are discussed has been
> discussed on StackOverflow (http://stackoverflow.com/
> questions/6981717/pythonic-way-to-combine-for-loop-and-if-statement)
> where it is suggested one uses a generator expression before the loop. None
> of these solutions seem very Pythonic to me.
> >
> > I appreciate there is a cost associated with changing the language
> syntax, and I do not understand all the finer details of the inner workings
> involved with the Python language development, however in my limited
> understanding in it would be:
> > - fully backwards compatible,
> > - require one to change "expression_list" to "or_test [comp_iter]" in
> the syntax of the for statement (if I got it right).
> > - it would mean there is a Pythonic solution to a current 'problem' that
> does not have one.
> >
> > A few problems I foresee:
> > - One wants for loops to bleed their target_list (that is the point
> normally), so this is different from generators,
> > - This allows for nesting of generators, like in a generator expression
> which might be hard to implement?
> >
> I think it might also be difficult to parse. Python currently supports
> this syntax:
>
> In [8]: for x in [1, 2] if True else [1, 2, 3]:
>    ...:     print(x)
>    ...:
> 1
> 2
>
> I'm not sure the parser could distinguish this structure from the one
> you propose (which would have a very different meaning) without making
> it significantly more complicated. Changes that make the parser more
> complicated than LL(1) have been consistently rejected in the past;
> see https://www.python.org/dev/peps/pep-3099/.
>
> >
> > Note that this has been suggested before at least once (
> https://mail.python.org/pipermail/python-dev/2007-November/075257.html),
> and that thread itself suggests it has been suggested before and shutdown
> by Guido (though no source is given for this).
> >
> > All the best,
> >
> > Henk-Jaap Wagenaar
> >
> > _______________________________________________
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>
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