In this guide, we'll take a look at how to check if a string contains a substring in Python. As usual, each approach we'll cover has different pros and cons.
The in Operator
The easiest way to check if a Python string contains a substring is to use the in
operator.
The in
operator is used to check data structures for membership in Python. It returns a Boolean [either True
or False
]. To check if a string contains
a substring in Python using the in
operator, we simply invoke it on the superstring:
fullstring = "StackAbuse"
substring = "tack"
if substring in fullstring:
print["Found!"]
else:
print["Not found!"]
This operator is shorthand for calling an object's __contains__
method, and also works well for checking if an item exists in a list. It's worth noting that it's not null-safe, so if our fullstring
was pointing to None
, an exception would be thrown:
TypeError: argument of type 'NoneType' is not iterable
To avoid this, you'll first want to check whether it points to None
or not:
fullstring = None
substring = "tack"
if fullstring != None and substring in fullstring:
print["Found!"]
else:
print["Not found!"]
The String.index[] Method
The String type in Python has a method called index[]
that can be used to find the starting index of the first occurrence of a substring in a string.
Check out our hands-on, practical guide to learning Git, with best-practices, industry-accepted standards, and included cheat sheet. Stop Googling Git commands and actually learn it!
If the substring is not found, a ValueError
exception is thrown, which can be
handled with a try-except-else block:
fullstring = "StackAbuse"
substring = "tack"
try:
fullstring.index[substring]
except ValueError:
print["Not found!"]
else:
print["Found!"]
This method is useful if you need to know the position of the substring, as opposed to just its existence within the full string.
The String.find[] Method
The String type has another method called find
which is more convenient to use than index[]
, because we don't need to worry about handling any exceptions.
If find[]
doesn't find a match, it returns -1, otherwise it returns the left-most index
of the substring in the larger string.
fullstring = "StackAbuse"
substring = "tack"
if fullstring.find[substring] != -1:
print["Found!"]
else:
print["Not found!"]
If you'd prefer to avoid the need to catch errors, then this method should be favored over index[]
.
Regular Expressions [RegEx]
Regular expressions provide a more flexible [albeit more complex] way to check strings for pattern matching. Python is shipped with a built-in module for regular expressions, called re
. The re
module contains a function called search[]
, which we can use to match a
substring pattern:
from re import search
fullstring = "StackAbuse"
substring = "tack"
if search[substring, fullstring]:
print "Found!"
else:
print "Not found!"
This method is best if you are needing a more complex matching function, like case insensitive matching. Otherwise the complication and slower speed of regex should be avoided for simple substring matching use-cases.
This article was written by Jacob Stopak, a software consultant and developer with passion for helping others improve their lives through code. Jacob is the creator of Initial Commit - a site dedicated to helping curious developers learn how their favorite programs are coded. Its featured project helps people learn Git at the code level.
One of the most common operations that programmers use on strings is to check whether a string contains some other string. If you are coming to Python from Java, for instance, you might have used the contains method to check if some substring
exists in another string. In Python, there are two ways to achieve this. The easiest way is via Python’s in operator. Let’s take a look at this example. As you can see, the in operator returns True when the substring exists in the string. Otherwise, it returns false. This method is very straightforward, clean, readable, and idiomatic. Another method you can use
is the string’s find method. Unlike the in operator which is evaluated to a boolean value, the find method returns an integer. This integer is essentially the index of the beginning of the substring if the substring exists, otherwise -1 is returned.First: Using the in operator
>>> str = "Messi is the best soccer player"
>>> "soccer" in str
True
>>> "football" in str
False
Second: Using the find method
Let’s see the find method in action.
>>> str = "Messi is the best soccer player"
>>> str.find["soccer"]
18
>>> str.find["Ronaldo"]
-1
>>> str.find["Messi"]
0
One cool thing about this method is you can optionally specify a start index and an end index to limit your search within.
For example
>>> str = "Messi is the best soccer player"
>>> str.find["soccer", 5, 25]
18
>>> str.find["Messi", 5, 25]
-1
Notice how a -1 was returned for “Messi” because you are limiting your search to the string between indices 5 and 25 only.
Python 3 Cheat Sheet for Beginners
Download a comprehensive cheat sheet for beginners with extensive code examples
that covers all the topics that you need to learn.
Some Advanced Stuff
Assume for a second that Python has no built-in functions or methods that would check if a string contains another string.
How would you write a function to do that?
Well, an easy way is to brute force by checking if the substring exists starting from every possible position in the original string.
For larger strings, this process can be really slow.
There are better algorithms for string searching.
I highly recommend this article from TopCoder if you want to learn more and dive deeper into string searching algorithms.
For more coverage of other string searching algorithms not covered in the previous article, this wikipedia page is great.
If you go through the previous articles and study them, your next question would be “well what algorithm does Python actually use?”
These kinds of questions almost always require digging into the source code.
But you are in luck because Python’s implementation is open source.
Alright, let’s dig into the code.
Perfect, I am happy the developers commented their code 🙂
It is very clear now that the find method uses a mix of boyer-moore and horspool algorithms.
Conclusion
You can use the in operator or the string’s find method to check if a string contains another string.
The in operator returns True if the substring exists in the string. Otherwise, it returns False.
The find method returns the index of the beginning of the substring if found, otherwise -1 is returned.
Python’s implementation [CPython] uses a mix of boyer-moore and horspool for string searching.
Learning Python?
Check out the Courses section!
Featured Posts
- The Python Learning Path [From Beginner to Mastery]
- Learn Computer Science [From Zero to Hero]
- Coding Interview Preparation Guide
- The Programmer’s Guide to Stock Market Investing
- How to Start Your Programming Blog?
Are you Beginning your Programming Career?
I provide my best content for beginners in the newsletter.
- Python tips for beginners, intermediate, and advanced levels.
- CS Career tips and advice.
- Special discounts on my premium courses when they launch.
And so much more…