Python nested for one-liner

Question

Let's say I have the following for loop:

d = defaultdict(int)
for f in foo:
    for b in f:
        d[b] += 1

I ripped my actual example of bloat which is unrelated to my question. Now I want to somehow shorten it. In e.g. list comprehensions we are allowed to use nested for loops, and if they're done properly they can be perfectly readable. So we reduce nesting and the amount of lines. My question here is, can we do nested for loops without doing the comprehension, so we would end with something like this, in Python?:

d[b] += 1 for b in for f in foo

Netwave · Accepted Answer · 2019-04-08 01:52:29Z

3

You could use itertools.chain:

for b in itertools.chain.from_iterable(foo): d[b] += 1

Notice that when trying to do this d[b] += 1 for b in for f in foo (which is of course invalid sintax) your intention is to mutate something, and that should not be done inside a comprehension.

edited Apr 8, 2019 at 1:52

answered Apr 8, 2019 at 1:46

Netwave

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4 Comments

iruvar Over a year ago

+1. Additionally, collections.Counter(itertools.chain.from_iterable(foo)) should simplify it even further

dabljues Over a year ago

What if I need to pass f to some other function, in my case: for b in windowed(f, 2)?

Netwave Over a year ago

@dabljues, I would stick to the nested for loop in that case. I think it would be the most readable thing to do.

dabljues Over a year ago

Sorry for my absence. Yeah, I guess you're right. Marking your answer as accepted, thank you!

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