Python Nested List

What is Python Nested List?

A list can contain any sort object, even another list (sublist), which in turn can contain sublists themselves, and so on. This is known as nested list.

You can use them to arrange data into hierarchical structures.

Create a Nested List

A nested list is created by placing a comma-separated sequence of sublists.

Example: Create a nested list

L = ['a', ['bb', ['ccc', 'ddd'], 'ee', 'ff'], 'g', 'h']

Access Nested List Items by Index

You can access individual items in a nested list using multiple indexes.

The indexes for the items in a nested list are illustrated as below:

Python Nested List Indexing

Example: Access items of a nested list

L = ['a', 'b', ['cc', 'dd', ['eee', 'fff']], 'g', 'h']
print(L[2])        # ['cc', 'dd', ['eee', 'fff']]
print(L[2][2])     # ['eee', 'fff']
print(L[2][2][0])  # eee

Negative List Indexing In a Nested List

You can access a nested list by negative indexing as well.

Negative indexes count backward from the end of the list. So, L[-1] refers to the last item, L[-2] is the second-last, and so on.

The negative indexes for the items in a nested list are illustrated as below:

Python Nested List Negative Indexing

Example: Access nested list items by Negative Index

L = ['a', 'b', ['cc', 'dd', ['eee', 'fff']], 'g', 'h']
print(L[-3])          # ['cc', 'dd', ['eee', 'fff']]
print(L[-3][-1])      # ['eee', 'fff']
print(L[-3][-1][-2])  # eee

Change Nested List Item Value

You can change the value of a specific item in a nested list by referring to its index number.

Example: change item value in a nested list

L = ['a', ['bb', 'cc'], 'd']
L[1][1] = 0
print(L)	# ['a', ['bb', 0], 'd']

Add items to a Nested list

To add new values to the end of the nested list, use append() method.

Example:

L = ['a', ['bb', 'cc'], 'd']
L[1].append('xx')
print(L)    # ['a', ['bb', 'cc', 'xx'], 'd']

When you want to insert an item at a specific position in a nested list, use insert() method.

Example:

L = ['a', ['bb', 'cc'], 'd']
L[1].insert(0,'xx')
print(L)    # ['a', ['xx', 'bb', 'cc'], 'd']

You can merge one list into another by using extend() method.

Example:

L = ['a', ['bb', 'cc'], 'd']
L[1].extend([1,2,3])
print(L)    # ['a', ['bb', 'cc', 1, 2, 3], 'd']

Remove items from a Nested List

If you know the index of the item you want, you can use pop() method. It modifies the list and returns the removed item.

Example:

L = ['a', ['bb', 'cc', 'dd'], 'e']
x = L[1].pop(1)
print(L)    # ['a', ['bb', 'dd'], 'e']

# removed item
print(x)	# cc

If you don’t need the removed value, use the del statement.

Example:

L = ['a', ['bb', 'cc', 'dd'], 'e']
del L[1][1]
print(L)    # ['a', ['bb', 'dd'], 'e']

If you’re not sure where the item is in the list, use remove() method to delete it by value.

Example:

L = ['a', ['bb', 'cc', 'dd'], 'e']
L[1].remove('cc')
print(L)    # ['a', ['bb', 'dd'], 'e']

Find Nested List Length

You can use the built-in len() function to find how many items a nested sublist has.

Example: Find length of a nested sublist

L = ['a', ['bb', 'cc'], 'd']
print(len(L))       # 3
print(len(L[1]))    # 2

Iterate through a Nested List

To iterate over the items of a nested list, use simple for loop.

Example: Iterate each item in a nested list using nested for loop

L = [[1, 2, 3],[4, 5, 6],[7, 8, 9]]   
for list in L:
    for number in list:
        print(number, end=' ')
# Output: 1 2 3 4 5 6 7 8 9