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How to Check if a String Contains a Specific Character in Python?

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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 did not find the substring.

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

if "secret" in raw_file_content:
    print("Found!")

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.

Generalize Your Check by Removing Case Sensitivity

By default, the ‘in’ operator is case-sensitive. This means that if you try to find the substring “secret” in the string “This is a secret message”, it won’t match and will return False. Python considers the lowercase ‘s’ and the uppercase ‘S’ as different characters.

If you want to perform a case-insensitive check, you can convert both the string and the substring to lowercase or uppercase using the ‘lower()’ method:

string = "This is a secret message"
substring = "secret"


if substring.lower() in string.lower():
    print("Found!")

In this example, both the string and the substring are converted to lowercase before performing the check. This ensures that the check is case-insensitive, and the message will be printed.

Learn More About the Substring

Python strings provide useful methods to manipulate and get information about substrings. Here are a few common methods that can help you work with substrings:

  • ‘startswith()’ method: Checks if a string starts with a specific substring.
  • ‘endswith()’ method: Checks if a string ends with a specific substring.
  • ‘index()’ method: Returns the index of the first occurrence of a substring in a string.
  • ‘count()’ method: Returns the number of occurrences of a substring in a string.
  • ‘replace()’ method: Replaces all occurrences of a substring with a different string.
  • ‘split()’ method: Splits a string into a list of substrings using a delimiter.

Find a Substring With Conditions Using Regex

The membership operator ‘in’ is great for simple substring checks. However, if you need to find substrings that match specific conditions, regular expressions can be a powerful tool.

Python provides the ‘re’ module for working with regular expressions. Here’s an example that uses regex to find all occurrences of a substring that starts and ends with a specific character:

import re


string = "This is a test string. [abc]def[ghi]jkl[mno]pqr"


pattern = r"\[.*?\]"
matches = re.findall(pattern, string)


for match in matches:
    print(match)

In this code snippet, the regex pattern ”[.*?]” is used to match all occurrences of a substring that starts with ’[’ and ends with ’]‘. The ‘findall()’ function from the ‘re’ module returns a list of all matches. The code then loops through the matches and prints each one.

Find a Substring in a pandas DataFrame Column

If you’re working with tabular data and need to find substrings in specific columns, using pandas can greatly simplify the task.

Assume you have a CSV file with a column named ‘text’. You can load the data into a pandas DataFrame and use the ‘str.contains()’ method to search for substrings in that column:

import pandas as pd


data = pd.read_csv('data.csv')


substring = "Python"
subset_df = data[data['text'].str.contains(substring)]


print(subset_df)

In this example, the ‘data.csv’ file is read into a pandas DataFrame. Then, the ‘str.contains()’ method is used to create a boolean mask of rows in the DataFrame where the ‘text’ column contains the substring “Python”. Finally, the boolean mask is used to filter the DataFrame, resulting in a new DataFrame that only contains the rows with the desired substring.

Key Takeaways

  • Python’s membership operator ‘in’ is the recommended way to check if a string contains another string. It returns True if the substring is found, and False otherwise.
  • You can use ‘in’ in conditional statements to make decisions based on the presence or absence of a substring.
  • To perform a case-insensitive check, you can convert both the string and the substring to lowercase or uppercase using the ‘lower()’ method.
  • Python strings provide various methods for working with substrings, such as ‘startswith()’, ‘endswith()’, ‘index()’, ‘count()’, ‘replace()’, and ‘split()‘.
  • If you need to find substrings that match specific conditions, regular expressions can be a powerful tool. Python’s ‘re’ module provides functions for working with regex.
  • If you’re working with tabular data, pandas offers convenient methods for searching for substrings in specific columns, such as ‘str.contains()’, ‘str.startswith()’, ‘str.endswith()’, and ‘str.extract()‘.

By understanding and using these techniques, you’ll be able to efficiently check for substrings in Python and perform actions based on their presence or absence.

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