[Python-ideas] Temporary variables in comprehensions

[Chris Barker - NOAA Federal]

> Setting a new comprehension variable is not likely to be free, and may even be
> more costly than calling f(x) twice if f() is a cheap expression:
>
>    [x+1 + some_func(x+1) for x in range(10)]
>
> could be faster than
>
>    [y + some_func(y) for x in range(10) let y = x + 1]

A bit of a nit — function call overhead is substantial in python, so
if that is an actual function, rather than a simple expression, it’ll
likely be slower to call it twice for any but trivially small
iterables.


>    [(y, y**2) let y = x+1 for x in (1, 2, 3, 4)]

Do we need the let?

[ g(y) for y = f(x) for c in seq]

Or, with expressions:

[y + y**2 for y = x+1 for x in (1,2,3)]

Maybe that would be ambiguous— I haven’t thought carefully about it.

-CHB