On Thu, May 31, 2018 at 2:55 PM, Chris Angelico <rosuav@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).

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]