Flatten list of lists [duplicate]

Question

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• Flattening a shallow list in Python [duplicate] 23 answers

I'm having a problem with square brackets in Python. I wrote a code that produces the following output:

[[180.0], [173.8], [164.2], [156.5], [147.2], [138.2]]

But I would like to perform some calculations with that, but the the square brackets won't let me.

How can I remove the brackets? I saw some examples to do that but I could not apply them to this case.

Answer 1

Flatten the list to "remove the brackets" using a nested list comprehension. This will un-nest each list stored in your list of lists!

list_of_lists = [[180.0], [173.8], [164.2], [156.5], [147.2], [138.2]]
flattened = [val for sublist in list_of_lists for val in sublist]

Nested list comprehensions evaluate in the same manner that they unwrap (i.e. add newline and tab for each new loop. So in this case:

flattened = [val for sublist in list_of_lists for val in sublist]

is equivalent to:

flattened = []
for sublist in list_of_lists:
    for val in sublist:
        flattened.append(val)

The big difference is that the list comp evaluates MUCH faster than the unraveled loop and eliminates the append calls!

If you have multiple items in a sublist the list comp will even flatten that. ie

>>> list_of_lists = [[180.0, 1, 2, 3], [173.8], [164.2], [156.5], [147.2], [138.2]]
>>> flattened  = [val for sublist in list_of_lists for val in sublist]
>>> flattened 
[180.0, 1, 2, 3, 173.8, 164.2, 156.5, 147.2,138.2]

Answer 2

I would use itertools.chain - this will also cater for > 1 element in each sublist:

from itertools import chain
list(chain.from_iterable([[180.0], [173.8], [164.2], [156.5], [147.2], [138.2]]))

Answer 3

Given

d = [[180.0], [173.8], [164.2], [156.5], [147.2], [138.2]]

and your specific question: How can I remove the brackets?

Using list comprehension :

new_d = [i[0] for i in d]

will give you this

[180.0, 173.8, 164.2, 156.5, 147.2, 138.2]

then you can access individual items with the appropriate index, e.g., new_d[0] will give you 180.0 etc which you can then use for math.

If you are going to have a collection of data, you will have some sort of bracket or parenthesis.

Note, this solution is aimed specifically at your question/problem, it doesn't provide a generalized solution. I.e., it will work for your case.

Answer 4

>>> lis=[[180.0], [173.8], [164.2], [156.5], [147.2], [138.2]]
>>> [x[0] for x in lis]
[180.0, 173.8, 164.2, 156.5, 147.2, 138.2]