[Python-ideas] Temporary variables in comprehensions

[fhsxfhsx]

As far as I can see, a comprehension like
alist = [f(x) for x in range(10)]
is better than a for-loop
for x in range(10):
  alist.append(f(x))
because the previous one shows every element of the list explicitly so that we don't need to handle `append` mentally.

But when it comes to something like
[f(x) + g(f(x)) for x in range(10)]
you find you have to sacrifice some readableness if you don't want two f(x) which might slow down your code.

Someone may argue that one can write
[y + g(y) for y in [f(x) for x in range(10)]]
but it's not as clear as to show what `y` is in a subsequent clause, not to say there'll be another temporary list built in the process.
We can even replace every comprehension with map and filter, but that would face the same problems.

In a word, what I'm arguing is that we need a way to assign temporary variables in a comprehension.
In my opinion, code like
[y + g(y) for x in range(10) **some syntax for `y=f(x)` here**]
is more natural than any solution we now have.
And that's why I pro the new syntax, it's clear, explicit and readable, and is nothing beyond the functionality of the present comprehensions so it's not complicated.

And I hope the discussion could focus more on whether we should allow assigning temporary variables in comprehensions rather than how to solve the specific example I mentioned above.
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