Do a function call in a list comprehension only once if needed twice [duplicate]

Question

Here a generic Python question about generators/list comprehension.

For a given iterable x I need a list comprehension which looks like this:

[ flatten(e) for e in x if flatten(e) != '' ]

The function flatten is potentially expensive, so it would be nice to call it only once. Is there a way to do this in an expressive one-liner?

Answer 1

Nest a generator:

[item for item in (flatten(e) for e in x) if item != '']

Or:

[item for item in map(flatten, x) if item != '']