Flatten list of list of tuples with list comprehension

Question

Why does the below list comprehension give an error? What am I missing?

a = [
[(1, 2), (11, 22), (111, 222)],
[(3, 4), (33, 44), (333, 444)],
[(5, 6), (55, 66), (555, 666)]
]

b = [k for k in j for j in i for i in a]
print(sorted(b))

I know there are more elegant/readable solutions, but this is for my own understanding.

Error: TypeError: 'int' object is not iterable

Desired Output: [1, 2, 3, 4, 5, 6, 11, 22, 33, 44, 55, 66, 111, 222, 333, 444, 555, 666]

Possible duplicate of [How to make a flat list out of list of lists?](https://stackoverflow.com/questions/952914/how-to-make-a-flat-list-out-of-list-of-lists) — mkrieger1, Feb 26 '19 at 10:53

score 3 · Accepted Answer · answered Feb 26 '19 at 10:52

The order in the list comprehension is wrong. Here is the correct solution:

In [5]: a = [
   ...: [(1, 2), (11, 22), (111, 222)],
   ...: [(3, 4), (33, 44), (333, 444)],
   ...: [(5, 6), (55, 66), (555, 666)]
   ...: ]
   ...: 
   ...: b = [j for x in a for y in x for j in y]
   ...: 
   ...: 
   ...: 

In [6]: 

In [6]: b
Out[6]: [1, 2, 11, 22, 111, 222, 3, 4, 33, 44, 333, 444, 5, 6, 55, 66, 555, 666]

In [7]: sorted(b) 
Out[7]: [1, 2, 3, 4, 5, 6, 11, 22, 33, 44, 55, 66, 111, 222, 333, 444, 555, 666]

Here is an SO discussion that explains nested list comprehension: Explanation of how nested list comprehension works?

Josh Friedlander · Answer 2 · 2019-02-26T11:05:35.243

One way to think about nested list comprehensions is to imagine them written out as a loop, and then scoop them up from top to bottom, finally using the last line at the beginning (since this is what we want to print). This way each variable is declared in memory, starting from the known and moving on to each temporary level.

So if we have:

for sublist in a:
    for tup in sublist:
        for item in tup:
            print(item)

We could rewrite this as

[print(item) for sublist in a for tup in sublist for item in tup]

> [1, 2, 11, 22, 111, 222, 3, 4, 33, 44, 333, 444, 5, 6, 55, 66, 555, 666]

(and so on ad infinitum)

Flatten list of list of tuples with list comprehension

2 Answers2