Python list comprehension - want to avoid repeated evaluation

Question

I have a list comprehension which approximates to:

[f(x) for x in l if f(x)] 

Where l is a list and f(x) is an expensive function which returns a list.

I want to avoid evaluating f(x) twice for every non-empty occurance of f(x). Is there some way to save its output within the list comprehension?

I could remove the final condition, generate the whole list and then prune it, but that seems wasteful.

Edit:

Two basic approaches have been suggested:

An inner generator comprehension:

[y for y in (f(x) for x in l) if y] 

or memoization.

I think the inner generator comprehension is elegant for the problem as stated. In actual fact I simplified the question to make it clear, I really want:

[g(x, f(x)) for x in l if f(x)] 

For this more complicated situation, I think memoization produces a cleaner end result.

Answer 1

[y for y in (f(x) for x in l) if y] 

Will do.