Nested List comprehension in Python

Question

I have a List inside of a List in Python and i want to convert them into a single list using List comprehension:

>>> aa = [[1,2],[1,2]]
>>> bb = [num for num in numbers for numbers in aa]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'numbers' is not defined
>>>

What am i doing wrong?

*The answer to my question isn't on the duplicate as stated above, it is below this question.

Possible duplicate of [Nested For Loops Using List Comprehension](http://stackoverflow.com/questions/3633140/nested-for-loops-using-list-comprehension) — khajvah, Oct 13 '15 at 06:57
[One more link](http://stackoverflow.com/questions/3927553/list-comprehension-in-nested-lists) — khajvah, Oct 13 '15 at 06:58
ya same question, but did u see the same answer on those questions that solved my problem? @khajvah — ellaRT, Oct 13 '15 at 07:00
btw, `itertools.chain(*aa)` does the same thing and is a lot more concise. — Eli Korvigo, Oct 13 '15 at 16:53

score 9 · Accepted Answer · answered Oct 13 '15 at 06:19

9

You have the for loops in your list comprehension in the opposite order -

bb = [num for numbers in aa for num in numbers]

Demo -

>>> aa = [[1,2],[1,2]]
>>> bb = [num for numbers in aa for num in numbers]
>>> bb
[1, 2, 1, 2]

answered Oct 13 '15 at 06:19

Anand S Kumar

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yes, yes. Thank you. that was confusing. I thought that i had it in the correct order. :) – ellaRT Oct 13 '15 at 06:20
python list comprehensions *are* confusing. unfortunately there is no decent alternative in the language. – WestCoastProjects Oct 21 '18 at 03:41

Nested List comprehension in Python

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