How to Fix – ValueError: substring not found

In Python, the ValueError: substring not found error occurs when you try to find a substring in a string using the index() function, but the substring is not present in the string. In this tutorial, we’ll a deeper look at this error and understand why this error occurs and how can we fix it.

Understanding the ValueError: substring not found error

The most common scenario in which this error occurs is when you’re trying to find the index of a substring inside a string using the built-in string index() function. If the substring is not present in the string, this error is raised.

Let’s take a look at the index() function.

The index() function in Python is used to find the index of the first occurrence of a substring in a given string. It returns the index of the first character of the substring if it is found in the string, otherwise, it raises a ValueError exception.

# create a string
string = "Peter Parker"
# index of the string "Parker"
index = string.index("Parker")
print(index)

Output:

6

In the above example, the index() function is used to find the index of the first occurrence of the substring “Parker” in the string “Peter Parker”. The function returns the value 6, which is the starting index of the first occurrence of the substring “Parker” in the string.

If you try to find the index of a substring not present in the string using the index() function, you’ll get an error.

# create a string
string = "Peter Parker"
# index of the string "Venom"
index = string.index("Venom")
print(index)

Output:

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

Cell In[5], line 4
      2 string = "Peter Parker"
      3 # index of the string "Venom"
----> 4 index = string.index("Venom")
      5 print(index)

ValueError: substring not found

In the above example, we try to find the index of the substring “Venom” in the string “Peter Parter”. Since this substring is not present in the given string, we get a `ValueError: substring not found” error.

How to fix the error?

Now that we know why this error occurs, let’s look at some ways in which we can resolve this error –

1. Using the string find() function
2. Using exception handling

Using the string find() function

The find() function in Python is a built-in string method that also returns the index of the first occurrence of a substring within a string but instead of giving an error if the substring is not present, it returns -1.

Let’s look at an example.

# create a string
string = "Peter Parker"
# index of the string "Venom"
index = string.find("Venom")
if index != -1:
    print("Substring found at index: ", index)
else:
    print("Substring not present")

Output:

Substring not present

In the above example, we are using the find() function to get the index of the substring “Venom” inside the string “Peter Parker”. You can see that we don’t get an error here.

Using exception handling

Alternatively, you can also use exception handling to tackle this error. Put the code where you’re trying to find the index of the substring using the index() function inside a try block and catch any ValueError that may occur.

Let’s look at an example.

# create a string
string = "Peter Parker"
# index of the string "Venom"
try:
    index = string.index("Venom")
    print("Substring found at index: ", index)
except ValueError:
    print("Substring not found")

Output:

Substring not found

We don’t get an error here because we’re catching the ValueError if it occurs and then printing out a message stating the substring was not found.

Conclusion

The ValueError: substring not found error occurs when you try to find a substring in a string using the string index() function, but the substring is not present in the string. To fix this error, you can use the string find() function instead that returns -1 if a substring is not present instead of raising an error. You can also use a try-except block to handle the error.

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