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How to check if a python string contains a substring?
In this python tutorial, we look at how you can check if a python string contains a substring. We look at the various methods and explain their use cases in detail.
This tutorial is a part of our initiative at Flexiple, to write short curated tutorials around often used or interesting concepts. However, in case you are here only for the solution use this link.
We check if a python string contains a substring for multiple reasons, however, its most commonly used in conditional statements. This runs a code in case a substring exists and another if the python string does not contain the substring. Another common use is to find the index of the substring in the python string.
If you are coming from another language, it most likely that you have come across the
contains function. Python also supports the __contains__ method, however, it also supports a few faster and more readable methods to check if a python string contains a substring.The in operator is the most easiest and pythonic method to check if a python string contains a substring. The in and not in are membership operators, they take in two arguments and evaluate if one is a member of another. They return True in case it is a member and False in case it isn't. And hence it is used to check if a substring is a member of a python string. This is an efficient alternative to the contains method and can also be used to check if an item exists in a list.
The
in is mainly used when the user only wants to check if it python string contains the substring and also because it is more readable. However, in case you are looking to return the index of the substring, the next solution is the goto.'substring` in stringThe syntax for
not in is the same. if "Hire" in "Hire the top freelancers":
print("Exists")
else:
print("Dose not exist")
#Output - ExistsThe
in operator is case sensitive, and the above code would've returned a false if the substring was "hire" and hence is a good practice to use it with the .lower() method. This method converts the string to lower case. As strings are immutable, this would not affect the original string.if "hire" in "Hire the top freelancers".lower():
print("Exists")
else:
print("Dose not exist")
#Output - ExistsPython comes with a few string methods that can be used to check if a python string contains a substring. We look at the
find() and Index() methods. These methods stand out from in as they return the index of the substring. However, they come with their cons we discuss them more in detail.The
string.index() method returns the starting index of the substring passed as a parameter. In this way, it could be used to check if a python string contains a substring. However, a major con is that in case the substring does not exist this method returns a ValueError and hence it needs to be placed inside a Try Except.Syntax of
index():string.index(value, start, stop)Here
string is the python string and value in the substring. The syntax also contains two optional parameters start and stop these are used in case you are looking for a substring within a particular index. Code using
index():try:
"Hire the top freelancers".index("Hire")
except ValueError:
print("Dose not exist")
else:
print(sting.index(sti))
#Output = 0Similar given
index() is case sensitive and using the .lower() is recommended.try:
"Hire the top freelancers".lower().index("hire")
except ValueError:
print("Dose not exist")
else:
print(sting.index(sti))
#Output = 0The
string.find() is another method that can be used to check if the python string contains the substring. Similar to index() , find() also returns the starting index of the substring. But unlike the index() method it does not return a ValueError in case the substring does not exist but rather returns -1 which is the index of the left-most string. Syntax of
find():string.find(value, start, end)The syntax and parameters of
find() are the same as index()Code using
find():if "Hire the top freelancers".find("Hire") != -1:
print("Hire the top freelancers".find("Hire"))
else:
print("Dose not exist")And again, since
find() is also case sensitive the following code can be used.if "Hire the top freelancers".lower().find("hire") != -1:
print("Hire the top freelancers".find("Hire"))
else:
print("Dose not exist").lower() methods as all the methods are case sensitive.index() method to ensure it is placed inside an try and except condition.19