how to apply BREAK for Itertools count in List Comprehensions?

Question

if this is duplicate, already answered then sorry, i didn't came across this question

as i was reading itertools count, generating an iterator using for loop is easy, and i was trying to do that in list comprehension but i am facing this problem

from itertools import *

by using for loop

for x in itertools.count(5,2): if x > 20: break else: print(x) 5 7 9 11 13 15 17 19

i tried to do this in list comprehension

[x if x<20 else break for x in count(5,2)] File "<ipython-input-9-436737c82775>", line 1 [x if x<20 else break for x in count(5,2)] ^ SyntaxError: invalid syntax

i tried with islice method and i got the answer

[x for x in itertools.islice(itertools.count(5),10)] [5, 6, 7, 8, 9, 10, 11, 12, 13, 14]

without using islice method, how can I exit(using break or any thing) by using only count method?

additionally how to implement break in list comprehensions?

Why don't you like islice? There's definitely no break inside list comprehension, but there's takewhile. — bereal
– bereal, Commented Oct 23, 2016 at 10:23
it's not about liking, i just want to know that "can i do in this way or not?" @bereal — avimatta
– avimatta, Commented Oct 23, 2016 at 10:26
i can use range, as i was reading itertools, i tried to do with itertools @falsetru — avimatta
– avimatta, Commented Oct 23, 2016 at 10:29

bereal · Accepted Answer · 2018-09-22 08:26:27Z

7

There's no break inside list comprehensions or generator expressions, but if you want to stop on a certain condition, you can use takewhile:

>>> from itertools import takewhile, count >>> list(takewhile(lambda x: x < 20, count(5, 2))) [5, 7, 9, 11, 13, 15, 17, 19]

edited Sep 22, 2018 at 8:26

answered Oct 23, 2016 at 10:27

bereal

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6 Comments

Padraic Cunningham Over a year ago

You can list(takewhile(int(20).__gt__, count(5, 2))) to remove the ugly lambda and it will be quite a but faster.

bereal Over a year ago

@PadraicCunningham faster yes, but I would perhaps question the comparative ugliness.

Padraic Cunningham Over a year ago

I meant ugly as much for the overhead as the visual representation, a lambda with any functional code is the kiss of death for performance which is generally what you are trying to achieve when using itertools, map, filter etc.

raratiru Over a year ago

@avimatta You can also accept the answer, this is the greatest "Thank you, it works!"

avimatta Over a year ago

@PadraicCunningham can you tell me "a lambda with any functional code is the kiss of death for performance?"

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