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 retu

Answer 1

How about defining:

def truths(L):
    """Return the elements of L that test true"""
    return [x for x in L if x]

So that, for example

> [wife.children for wife in henry8.wives]
[[Mary1], [Elizabeth1], [Edward6], [], [], []]

> truths(wife.children for wife in henry8.wives) 
[[Mary1], [Elizabeth1], [Edward6]]