Remove items from a list while iterating in python

A python list is a collection of items of same or different types.
This article will show different methods to remove items from a list while iterating in python.
The item will be deleted based on some condition.

1. Using for loop and range function

Iterate over the list backwards and remove the element if it matches certain condition.
Below code declares a list of string elements. We need to remove elements that start with letter ‘d’.

It iterates over the list using for loop and range function backwards.
In every iteration, it checks if the current element starts with ‘d’ and removes it.
Element is removed using del function of list. It takes an element as argument and removes it from the list.
Also, to check if list element begins with letter ‘d’, startswith function is used.

# declare list
element_list = ['abc', 'def', 'dfv', 'acb', 'xyz']
print("List before deletion:", element_list)
# iterate over the list
for i in range(len(element_list) - 1, -1, -1):
    # check if element begins with 'd'
    if element_list[i].startswith('d'):
        # remove it
        del(element_list[i])
# print list after deletion
print("List after deletion:", element_list)

range function takes three arguments.
First is the start index which is [length of list – 1], that is, the index of last list element(since index of list elements starts from 0 till length – 1).
Second argument is the index at which to stop iteration.
Since we need to compare all elements, this value must be -1. If this value is 0, then first element will not be checked.
Third argument is the step size.
Since we need to decrease index by 1 in every iteration, this should be -1.

Output is

Method 2: Using while loop

Above method iterates over the list backwards.
If you need to iterate it in forward direction, the use a while loop as shown below.

# declare list
element_list = ['abc', 'def', 'dfv', 'acb', 'xyz']
print("List before deletion:", element_list)
# list index variable
index = 0
# iterate over the list
while index < len(element_list):
    # check if element begins with 'd'
    if element_list[index].startswith('d'):
        # remove it
        del(element_list[index])
    else:
        index += 1
# print list after deletion
print("List after deletion:", element_list)

It iterates over the list using a while loop starting from first till last element.
In every iteration, it checks the condition and removes the element if the condition matches otherwise loop index is incremented for the next element.

Note that if the element is removed, loop index is not incremented. This is because when the element is removed, index of remaining elements are reduced by 1.
In this case, if we increment the loop index, then some elements may be skipped from comparison.
Output of execution is

Other methods

For deleting an element from the list, instead of using del function, following two functions can also be used.
1. remove()

This method of list class can also be used to remove an item from the list.
This method takes the element to be removed from the list and removes it from the list.
Thus, in place of del function, this method can be used as element_list.remove(element_list[index])

2. pop()

This method of list class takes the index of a list item and removes the item at that index.
It can be used as element_list.pop(index)

Hope the article was useful.