CODEWITHSUNDEEP
pythonlist-comprehension
damian
damian

Reputation: 165

Nested list comprehension / merging nested lists

I have a problem understanding a nested list comprehension structure.

I have a list

>>> test
[[1, 2, 3], [4, 5], [6, 7, 8]]

If I do

t2=[]
for x in test:
    for y in x:
        t2.append(y)

then it returns 

>>> t2
[1, 2, 3, 4, 5, 6, 7, 8]

which is exactly what I want. But WHY can't I do

t3=[y for y in x for x in test]

This gives me

>>> t3
[6, 6, 6, 7, 7, 7, 8, 8, 8]

Can anybody explain to me why t3 is not the same as t2? How ca I write a list comprehension expression that gives me the same as t2? Thank you very much for your help!

Upvotes: 2

Views: 506

Answers (4)

dawg
dawg

Reputation: 104102

Just making sure you know about itertools chain:

>>> test=[[1, 2, 3], [4, 5], [6, 7, 8]]
>>> from itertools import chain
>>> list(chain(*test))
[1, 2, 3, 4, 5, 6, 7, 8]

Upvotes: 0

njzk2
njzk2

Reputation: 39403

In your code, before starting, x = [6, 7, 8] from your previous loop (as pointed out by jonsharpe).

Therefore, it unfolds as such:

for y in x:
    for x in test:
        t3.append(y)

x in the first loop point to [6, 7, 8], and is later reassigned, but that does not change the reference that is used in the first loop. The result would be the same if the second x had a distinct name.

Upvotes: 1

jonrsharpe
jonrsharpe

Reputation: 122157

You need to reverse the for loops:

t3 = [y for x in test for y in x]

otherwise (if you don't run the multi-line version beforehand!) x is undefined. Your code only ran on a fluke - x was still what it was at the end of the previous for loop, hence your results.

Upvotes: 0

user2555451
user2555451

Reputation:

The for ... in ... clauses inside a list comprehension need to go in the same order as if they were normal for-loops:

>>> test = [[1, 2, 3], [4, 5], [6, 7, 8]]
>>> t3 = [y for x in test for y in x]
>>> t3
[1, 2, 3, 4, 5, 6, 7, 8]
>>>

Upvotes: 3

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