How to flatten a list of lists in Python
Converting a list of lists (2D), into a list (1D) is called flattening. There are several approaches to this problem. We will look at a few important ones.
Approaches to flattening lists
Nested loops
This is perhaps the most intuitive approach to flattening. We pick every element from the list of lists and put it into our flat list.
List_2D = [[1,2,3],[4,5,6],[7,8,9]] #List to be flattenedList_flat = []for i in range(len(List_2D)): #Traversing through the main listfor j in range (len(List_2D[i])): #Traversing through each sublistList_flat.append(List_2D[i][j]) #Appending elements into our flat_listprint("Original List:",List_2D)print("Flattened List:",List_flat)
Functools (reduce-iconcat)
Perhaps the fastest execution can be achieved through this approach. However, it isn’t very intuitive and requires importing a few libraries.
import functoolsimport operatorList_2D = [[1,2,3],[4,5,6],[7,8,9]] #List to be flattenedList_flat = functools.reduce(operator.iconcat, List_2D, [])print("Original List:",List_2D)print("Flattened List:",List_flat)
Itertools(chain)
This is another notable approach. Although not as fast as the previous one, it’s still faster than using Nested-loops.
import itertoolsList_2D = [[1,2,3],[4,5,6],[7,8,9]] #List to be flattenedList_flat = list(itertools.chain(*List_2D))print("Original List:",List_2D)print("Flattened List:",List_flat)
Other approaches
The approaches we discussed so far are a few of the fastest known ones. However, there are other approaches to the problem, including:
- Functools - reduce
- NumPy - concatenate
- NumPy - flat
- Summing inner lists
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