On Thu, May 31, 2018 at 11:23 PM, Peter O'Connor <peter.ed.ocon...@gmail.com> wrote: > On Thu, May 31, 2018 at 2:55 PM, Chris Angelico <ros...@gmail.com> wrote: >> >> [process(tx, y) for x in xs for tx in [transform(x)] for y in yz] >> >> ... >> >> I think Serhiy was trying to establish this form as a standard idiom, >> with optimization in the interpreter to avoid constructing a list and >> iterating over it (so it would be functionally identical to actual >> assignment). I'd rather see that happen than the creation of a messy >> 'given' syntax. > > > Perhaps it wouldn't be crazy to have "with name=initial" be that idiom > instead of "for name in [initial]". As .. > > [process(tx, y) for x in xs with tx=transform(x) for y in yz] > > .. seems to convey the intention more clearly. More generally (outside of > just comprehensions), "with name=expr:" could be used to temporarily bind > "name" to "expr" inside the scope of the with-statement (and unbind it at > the end).
Except that 'with' means context managers, not just assignment. Also, it's not backward-compatible; if the "for var in [val]" syntax becomes an accepted idiom, it'll be valid in all versions of Python back to, what, 2.4? and just won't be optimized in older versions. Making a new syntax misses out on that benefit, so it needs to be a really good syntax, and 'with' isn't. > And then I could have my precious initialized generators (which I believe > cannot be nicely implemented with ":=" unless we initialize the variable > outside of the scope of the comprehension, which introduces the problem of > unintended side-effects). > > smooth_signal = [average with average=0 for x in seq with > average=(1-decay)*average + decay*x] I want to read this as "average with average=0" blah blah, which doesn't make a lot of sense. It'd be far FAR better to mess with things so that external assignment works. ChrisA _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
