5 Python methods to check if a substring exists in a Given string

In this article, we will briefly discuss about the different python methods that we can use to check if a substring is present inside a string along with some shortcuts that can make our coding efficient.

Overview of Strings in Python

Strings are an array of Unicode characters occupying a storage of few bytes.

If you come from a different language, there are high chances of your interaction with the char keyword, but in python there is no such char keyword and thus we can say that a single letter inside quotation marks in python is a string with length 1.

Strings behave the same way as like arrays and thus can be accessed by using the [] operator.

A substring is a sequence of Unicode characters within a string.

Examples of Strings in python:

‘python’ , ‘coding’ , ‘py’ , ‘d’ , ‘xyz’ , ‘Python is a good language’

Different Substring Check methods in Python

Let’s start exploring the various python methods starting with the powerful and most common in operator.

Using the in operator

The in operator is one of the fastest and most generic method adopted by python programmers as they know the power of the in operator and its value in python.

The in operator evaluates the presence of a substring inside a string and return a boolean value accordingly.

main_string = "Python is a good programming language"
sub_string1 = "good"
sub_string2 = "not"

if sub_string1 in main_string:
    print("Substring Found !")
else:
    print("Substring not Found !")
    

print(sub_string2 in main_string)

OUUTPUT:

Substring Found !
False

Thus, we would now have a fair idea as of why in is so valuable in python.

Using the find() method

Python’s default find() method is used to check if a substring is present in a string and this method looks for the lowest index at which the substring is found.

It returns the particular index if the substring is present else returns -1 if substring is not present.

main_string = "Python is a good programming language"
sub_string1 = "good"
sub_string2 = "not"

isFound = main_string.find(sub_string1)

if isFound >= 0:
    print("Substring Found !")
else:
    print("Substring not Found !")

print(main_string.find(sub_string2))

OUTPUT:

Substring Found !
-1

Other than the generic method of using find(), we can also try out some cool in-built features of it.

The cool feature is that we can specify starting and ending indices for our search, in case of a multiline main string.

Syntax:

print( main_string.find(sub_string , starting index , ending index ))
main_string = "Python is a good programming language"

sub_string1 = "good"

print(main_string.find(sub_string1,1,10))
print(main_string.find(sub_string1,10,30))

OUTPUT:

-1
12

As discussed, we can clearly see that since the index of good is 12 therefore when we specify indices from 1-10, we get -1 as result but when we search in indices from 10-30, we get the output as 12.

Using the count() method

The count() is one of those generic python programming methods which return binary value as output.

This method searches for the occurrence of a substring inside a string and returns 1 if substring is present else returns 0 when substring is not present.

main_string = "Python is a good programming language"

sub_string1 = "good"
sub_string2 = "not"

isExists = main_string.count(sub_string1)

if isExists == 1 :
    print("Substring Found !")
else:
    print("Substring not Found !")

print(main_string.count(sub_string2))

OUTPUT:

Substring Found !
0

Using the index() method

The index() method is also one of the unique methods of achieving the result.

Unlike other methods, this doesn’t return a value as output but instead it returns a ValueError if a substring is not present inside a string & if a substring is present then it returns the lowest index of that substring.

Hence, Handling exception is the only way to evaluate the existence of a substring inside of a string.

main_string = "Python is a good programming language"

sub_string1 = "good"
sub_string2 = "not"

try:
    result = main_string.index(sub_string1)
    print("Substring Found !")
except ValueError as e:
    print("Error Code:",e)

try:
    result = main_string.index(sub_string2)
    print("Substring Found")
except ValueError as e:
    print("Error Code:",e)

OUTPUT:

Substring Found !
Error Code: substring not found

Using the operator.contains() method

The hidden method / barely used method for searching substring inside a string is the operator.contains() method.

But this method can be effective at times. Implementation of this method requires the operator function which has to be imported explicitly in the code.

This method returns a boolean value as output depending the presence of a substring inside a string.

import operator

main_string = "Python is a good programming language"

sub_string1 = "good"
sub_string2 = "not"
 

if operator.contains(main_string,sub_string1) :
    print("Substring Found !")
else:
    print("Substring not Found !")

print(operator.contains(main_string,sub_string2))

OUTPUT:

Substring Found !
False

Since, we have discussed all the methods for evaluating the existence of a substring inside a string now it’s the time to discuss the performance of all our methods and which method should we use in order to make our programming faster.

Discussing Performance

Now we will see the performance aspects of different methods

import timeit
code_setup = """
main_string = "Python is a good programming language"
sub_1 = "good"
"""

## With in operator
code_1 = """
if sub_1 in main_string:
    # print("Substring Found !")
    pass
else:
    # print("Substring not Found !")
    pass
"""

## With find method
code_2 = """
result = main_string.find(sub_1)

if result >= 0:
    # print("Substring Found !")
    pass
else:
    # print("Substring not Found !")
    pass

"""

## With count method
code_3 = """
result = main_string.count(sub_1)

if result == 1 :
    # print("Substring Found !")
    pass
else:
    # print("Substring not Found !")
    pass

"""

## With index method  
code_4 = """
try:
    result = main_string.index(sub_1)
    # print("Substring Found !")
    pass
except ValueError as e:
    # print("Error Code:",e)
    pass

"""

## With Contains Method
code_5 = """
import operator

if operator.contains(main_string,sub_1) :
    # print("Substring Found !")
    pass
else:
    # print("Substring not Found !")
    pass
"""
print('Time With in operator: ', timeit.timeit(setup = code_setup , stmt= code_1, number= 10000000))
print('Time With find method: ', timeit.timeit(setup = code_setup , stmt= code_2, number= 10000000))
print('Time With count method: ', timeit.timeit(setup = code_setup , stmt= code_3, number= 10000000))
print('Time With index method: ', timeit.timeit(setup = code_setup , stmt= code_4, number= 10000000))
print('Time With Contains Method: ', timeit.timeit(setup = code_setup , stmt= code_5, number= 10000000))


OUTPUT:

Time With in operator:  0.5566656999999999
Time With find method:  2.0406945999999997
Time With count method:  2.0501952000000006
Time With index method:  1.8084388000000002
Time With Contains Method:  2.2768479

From the output, it is clearly visible that our mighty in operator stands out above all other methods and hence in adds one more time-saving usability into itself.

So as a python programmer, we should always be aware that in can solve many of our searching tasks.

Conclusion

In this article, we discussed in detail about all the various methods with the help of which we can easily search for a substring inside a string.

In the later part, we discussed about the performance of all the methods and came up with the most preferable method that can be used by programmers to solve this problem.

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I am Dibyanshu Mohanty, Undergraduate Engineering Student at VIT, Vellore with a keen interest in Web and App Development

https://python.shiksha/authors/dmohanty/