How to Use ‘Contains’ in Python: A Comprehensive Guide

Python is a popular programming language that offers a range of functions for developers. One of its most commonly used functions is "contains," which allows developers to check whether a particular substring or character is present within a given string. This guide will provide an in-depth understanding of how to use "contains" in Python, along with examples and best practices.

Table of Contents

What is "contains"?

"Contains" is a function in Python that enables developers to check whether a particular string is present within another string. The function searches for the specified substring or character within the given string and returns either True or False depending on whether the search string is found.

The syntax for the "contains" function is as follows:


Here, "string" is the string in which the search function is performed, and "substring" is the string that we are searching for in the "string."

Basic Example of "contains"

Let’s take a basic example to understand how the "contains" function works in Python:

text = "This is a sample text"
if "sample" in text:
    print("The text contains the word 'sample'")
    print("The text doesn't contain the word 'sample'")

Here, we first define a string "text" that contains the phrase "This is a sample text." We then use the "in" operator to check whether the substring "sample" is present in the "text" string. If it is present, we print a message that says the text contains the word "sample." If it is not present, we print a message that says the text doesn’t contain the word "sample."

Advanced Example of "contains"

Let’s take an advanced example to understand how the "contains" function can be used in real-world applications. Consider the following code:

def check_username(username):
    if username.contains("admin"):
        print("Username rejected. Please enter a different username.")
        print("Username accepted.")

Here, we have defined a function "check_username" that takes a username as input. The function checks if the username contains the substring "admin." If it does, the function prints a message that says the username is rejected and advises the user to enter a different username. If the username does not contain the substring "admin," the function prints a message that says the username is accepted.

Best Practices for Using "contains" in Python

When using the "contains" function in Python, there are a few best practices to keep in mind:

1. Use the "in" Operator

The "in" operator is the most commonly used method for checking whether a substring is present in a given string. It is highly recommended to use the "in" operator instead of the "contains" function as it is more efficient, and the code is easier to read.

2. Case Sensitivity

The "contains" function is case-sensitive. This means that if the substring is in uppercase and the string is in lowercase, the function will not be able to find the substring. To avoid this issue, you can convert both the string and the substring to the same case using the "lower()" or "upper()" function.

3. Use Regular Expressions

If you need to perform more complex searches, you can use regular expressions to search for patterns within a string. Regular expressions provide a powerful and flexible way to search for substrings in a text.

4. Avoid using "not in"

While the "not in" operator is an efficient way to check whether a substring is not present in a string, it can be misleading at times. For example, if you have a string "Python is great," and you want to check whether the string "is not" is present, using "not in" will return True, although the substring "is not" is present in the string. It is recommended to use "in" instead of "not in" whenever possible.

5. Use Error Handling

When using the "contains" function, it is essential to use error handling to avoid any unexpected issues that may arise. For example, if the substring you are searching for is not present in the string, the function will return an error. To avoid this, you can use try and except statements to handle the error and print a message to the user.


The "contains" function in Python is a powerful tool that allows developers to search for substrings in a given string. By using the "in" operator and following best practices such as case sensitivity, regular expressions, and error handling, developers can ensure that their code is efficient and accurate. By incorporating these practices into your Python code, you can make the most of the "contains" function and take your programming skills to the next level.

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