Python list comprehension: test function return

Question

Is there a way to test the return of a function in a list (or dict) comprehension? I'd like to avoid writing that:

lst = []
for x in range(10):
  bar = foo(x)
  if bar:
    lst.append(bar)

and use a list comprehension instead. Obviously, I don't want to write:

[foo(x) for x in range(10) if foo(x)]

so?

[foo(x) for x in range(10) if ??? ]

Why don't you want to use the first way? It's the appropriate way. — Rohit Jain, Nov 29 '12 at 18:28
@Rohit Jain -- depending on `foo`, it could be an expensive operation which you don't want to do twice. — mgilson, Nov 29 '12 at 18:30
@TimPietzcker -- Ahh... I thought Rohit was refering to the first list-comprehension... I'm following the comment stream now. Thanks. — mgilson, Nov 29 '12 at 18:34
@TimPietzcker.. Yeah I was talking about 1st list comprehension only. — Rohit Jain, Nov 29 '12 at 18:35
@TimPietzcker -- Argv! As soon as I think I get to the same page as everyone else, I find out they were on page I just left. :) — mgilson, Nov 29 '12 at 18:37
For various reasons: - for equal readability, I prefer conciseness - list comprehension are faster - intellectual stimulation — Adrien, Nov 29 '12 at 18:42

NPE · Accepted Answer · 2012-11-29T18:34:01.487

10

How about

filter(None, map(foo, range(10)))

If you don't want to keep the intermediate list, replace map() with itertools.imap(). And with itertools.ifilter(), the whole thing could be turned into a generator.

itertools.ifilter(None, itertools.imap(foo, range(10)))

edited Nov 29 '12 at 18:34

answered Nov 29 '12 at 18:28

NPE

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Or, rather than `imap`, you could use a genexp: `filter(None,(foo(x) for x in range(10)))` – mgilson Nov 29 '12 at 18:32

moooeeeep · Answer 2 · 2018-01-09T10:55:45.710

Just make a generator to compute the values and build the filtered list from the generator afterwards.

Example:

# create generator via generator expression
results = (foo(x) for x in xrange(10))
# build result list, not including falsy values
filtered_results = [i for i in results if i]

For reference:

Python list comprehension: test function return

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