Add element to List if not already exists using Python

This tutorial will discuss about unique ways to add element to list if not already present in List.

Table Of Contents

Introduction to not in Operator in Python

In Python, we have an operator not in, that we can apply on an element and a sequence. Like this,

Let’s see the complete example,

element not in sequence

If checks for the existance of element in the given sequence, and returns True if the given element does not exist in the sequence. Whereas, it returns False if the given element is present in the given sequence. This sequence can be a list, set , tuple or any other iterable sequence in Python.

Using ‘not in’ Operator to append element if not already present in Python List

To check if an element is present in a list or not, we can apply the not in operator on the element and the list. If it returns True, then it means element is present in the list, otherwise element is not present in the list. So, if given element is not present in list, then append it to the list using the append() function.

This approach is usefull, if we want to keep only unique elements in a list, just like a set.

Example 1

In this example, we have a list of strings, and we will add a new string (‘the’) in it. But before that, we will check if the given string exists in the list or not, and if it is not present in the list, then only we will add the element to it.

Let’s see the complete example,

listOfStrings = ['is', 'are', 'at', 'why', 'what']

# Element to be added
value = 'the'

# Add item to list if not already present in list
if value not in listOfStrings:
    listOfStrings.append(value)

print(listOfStrings)

Output

['is', 'are', 'at', 'why', 'what', 'the']

As the string ‘the’ was not already present in the list, therefore it got added in list.

Example 2

In this example, we have a list of strings, and we will try to add a new string (‘at’) in it, but if and only if it is not already present in the list.

Let’s see the complete example,

listOfStrings = ['is', 'are', 'at', 'why', 'what']

# Element to be added
value = 'at'

# Add item to list if not already present in list
if value not in listOfStrings:
    listOfStrings.append(value)

print(listOfStrings)

Output

['is', 'are', 'at', 'why', 'what']

As the string at was already present in the list, therefore it skipped adding it again in the list.

Summary

We learned about a unique to add an element to list if not already present in List. Thanks.