Skip to content

Effortlessly Manipulate Python Strings: Explained with str.contains


How to Check if a Python String Contains a Substring

If you’re new to programming or come from a programming language other than Python, you may be looking for the best way to check whether a string contains another string in Python.

Identifying such substrings comes in handy when you’re working with text content from a file or after you’ve received user input. You may want to perform different actions in your program depending on whether a substring is present or not.

In this tutorial, you’ll focus on the most Pythonic way to tackle this task, using the membership operator in. Additionally, you’ll learn how to identify the right string methods for related, but different, use cases.

Finally, you’ll also learn how to find substrings in pandas columns. This is helpful if you need to search through data from a CSV file. You could use the approach that you’ll learn in the next section, but if you’re working with tabular data, it’s best to load the data into a pandas DataFrame and search for substrings in pandas.

How to Confirm That a Python String Contains Another String

If you need to check whether a string contains a substring, use Python’s membership operator in. In Python, this is the recommended way to confirm the existence of a substring in a string:

raw_file_content = """Hi there and welcome.
This is a special hidden file with a SECRET secret.
I don't want to tell you The Secret,
but I do want to secretly tell you that I have one."""
"secret" in raw_file_content

The in membership operator gives you a quick and readable way to check whether a substring is present in a string. You may notice that the line of code almost reads like English.

Note: If you want to check whether the substring is not in the string, then you can use not in:

"secret" not in raw_file_content

Because the substring “secret” is present in raw_file_content, the not in operator returns False.

When you use in, the expression returns a Boolean value:

  • True if Python found the substring
  • False if Python didn’t find the substring

You can use this intuitive syntax in conditional statements to make decisions in your code:

if "secret" in raw_file_content:

In this code snippet, you use the membership operator to check whether "secret" is a substring of raw_file_content. If it is, then you’ll print a message to the terminal.