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Python Program to Flatten a List without using Recursion

  • Last Updated : 03 Jan, 2021
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The task is to convert a nested list into a single list in Python i.e no matter how many levels of nesting are there in the Python list, all the nested has to be removed in order to convert it to a single containing all the values of all the lists inside the outermost brackets but without any brackets inside. In other words, an element in a list can be a number or a list. It needs to be converted to a list where each element in the list is only a number.


Input: [1,2,[3,4,[5,6],7],[[[8,9],10]],[11,[12,13]]]
Output: [1,2,3,4,5,6,7,8,9,11,12,13]

Input: [1, [2, 3]]

Output: [1, 2, 3]

There are two methods to do this without recursion.

Method 1: Using a stack


  • Initialization: Push the main list in a stack
  • Make an empty list to store the final result (result variable)
  • Loop till stack is not empty
    1. Pop the last added element of the stack and store it (current variable)
    2. If the popped element is a list than pushing all the elements of that list in the stack
    3. If the popped element is not a list, append that element in the result
  • Reverse the result list to get the final output in the original list’s order

Below is the implementation:


# Input list
l = [1, 2, [3, 4, [5, 6], 7],
     [[[8, 9], 10]],
     [11, [12, 13]]]
# Function to flatten the the list
def flatten(input_list):
    # Final list to be returned
    result = []
    # Creating stack and adding 
    # all elements into it
    stack = [input_list]
    # Iterate stack till it
    # does not get empty
    while stack:
        # Get the last element of the stack
        current = stack.pop(-1)
        # If the last element is a list,
        # add all the elements of that list in stack
        if isinstance(current, list):
        # Else add that element in the result
    # Reverse the result to get the
    # list in original order
    return result
# output list
ans = flatten(l)


[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

Method 2: Using libraries

The package iteration_utilities can also be used to flatten a list.


from iteration_utilities import deepflatten
l = [1, 2, [3, 4, [5, 6], 7], 
     [[[8, 9], 10]], [11, [12, 13]]]
ans = list(deepflatten(l))


[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

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