Checking substrings in Python: Methods and examples

What is the best way to check whether a string contains a specific substring in Python?

Python has several ways to check whether a string contains a specific substring. Each approach has its advantages and is suitable for different applications. Here are some common methods to check for substrings in a string:

Using the in keyword:

The in keyword can be used to check if a substring is present in a string. It returns a Boolean value (True or False) indicating whether the substring exists.

Code:

str1 = "Good, Morning!"
# Check if the substring exists
if "Morning" in str1:
    print("Substring found.")
else:
    print("Substring not found.")

Output:

Substring found.

Using the str.find() method:

The str.find() method is used to get the index of the first occurrence of the substring in the string. If the substring is not found, it returns -1.

Code:

my_string = "Good Morning!"
# Check if the substring exists and get its index
index = my_string.find("Morning")
if index != -1:
    print(f"Substring found at index {index}.")
else:
    print("Substring not found.")

Output:

Substring found at index 5.

Using the str.index() method:

The str.index() method is similar to str.find(), but if the substring is not found, it raises a ValueError. When using this method, it is essential to handle exceptions.

Code:

str1 = "Good, Morning!"

try:
    index = str1.index("Morning")
    print(f"Substring found at index {index}.")
except ValueError:
    print("Substring not found.")
Output:
Substring found at index 6.

Code:

str1 = "Good, Morning!"

try:
    index = str1.index("Morningg")
    print(f"Substring found at index {index}.")
except ValueError:
    print("Substring not found.")

Output:

Substring not found.

Using Regular Expressions (re module):

Using regular expressions from the re module, we can check for substrings in more complex patterns.

Code:

import re
str1 = "Good, Morning!"
# Use a regular expression to check for the substring
if re.search(r"Morning", str1):
    print("Substring found.")
else:
    print("Substring not found.")

Output:

Substring found.

Using String Slicing:

You can use string slicing to check if a substring exists by extracting a part of the string.

Code:

str1 = "Good, Morning!"

# Check if the substring exists using string slicing
sub_string = "Morning"
if sub_string in str1[:len(str1)]:
    print("Substring found.")
else:
    print("Substring not found.")