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 — khajvah
– khajvah, Commented Oct 13, 2015 at 6:57
ya same question, but did u see the same answer on those questions that solved my problem? @khajvah — ellaRT
– ellaRT, Commented Oct 13, 2015 at 7:00

Anand S Kumar · Accepted Answer · 2015-10-13 06:19:22Z

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, 2015 at 6:19

Anand S Kumar

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2 Comments

ellaRT Over a year ago

yes, yes. Thank you. that was confusing. I thought that i had it in the correct order. :)

WestCoastProjects Over a year ago

python list comprehensions are confusing. unfortunately there is no decent alternative in the language.

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