How to Fix ValueError Exceptions in Python

The Python ValueError is an exception that occurs when a function receives an argument of the correct data type but an inappropriate value. This error usually occurs in mathematical operations that require a certain kind of value.

To use an analogy, imagine trying to enroll an adult in a children's school. The person is a valid human, but their age is inappropriate for the setting. Similarly, a ValueError occurs when a function receives the correct type of input but with an unsuitable value.

What Causes ValueError

The Python ValueError is raised when an object is assigned the right data type but the wrong value for a certain operation. Some of the most common scenarios where this can happen are:

• If the value is invalid for the operation. For example, if a negative integer is passed to a square root operation.
• Performing an operation when the value does not exist. For example, trying to remove a value from a list where it does not exist.
• Trying to unpack more values than available. For example, trying to unpack values from a list with 5 items to 3 variables.

ValueError Examples

Example One

Here’s an example of a Python ValueError raised when trying to perform a square root operation on a negative number:

import math

math.sqrt(-100) #Performing square root operation on negative number

In the above example, a negative integer is passed to the math.sqrt() function. Since the function expects a positive integer, running the above code raises a ValueError:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    math.sqrt(-100)
ValueError: math domain error

Example Two

Here’s an example of a Python ValueError raised when trying to remove a value from a list where it does not exist:

myString = "Hello"
myList = ["World"]
myList.remove(myString) #Trying to remove value from list that does not contain it

In the above example, the variable myString with a value of "Hello" is attempted to be removed from the list myList. Since myList does not contain the value, running the above code raises a ValueError:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    myList.remove(myString) #Trying to remove value from list that does not contain it
ValueError: list.remove(x): x not in list

Example Three

Here’s an example of a Python ValueError raised when trying to unpack more values from a list than available:

myList = [10, 9, 8, 7, 6, 5]
i,j,k = myList #Trying to unpack more values than available variables

In the above example, the list myList containing 5 values is attempted to be unpacked and assigned to 3 variables. Since the number of values exceed the number of available variables, running the above code raises a ValueError:

Traceback (most recent call last):
  File "test.py", line 2, in <module>
    i,j,k = myList
ValueError: too many values to unpack (expected 3)

How to Fix ValueError Exceptions

To resolve the ValueError in Python code, a try-except block can be used. The lines of code that can throw the ValueError should be placed in the try block, and the except block can catch and handle the error.

Using this approach, the previous examples can be updated to handle the error:

Example One

Code:

import math

try: #Placing operation in try-except block
    math.sqrt(-100)
except ValueError:
    print('Positive number expected for square root operation')

Here, a check is performed for the ValueError using the try-except block. When the above code is executed, the except block catches the ValueError and handles it, producing the following output:

Positive number expected for square root operation

Example Two

Code:

myString = "Hello"
myList = ["World"]

try: #Placing operation in try-except block
    myList.remove(myString)
except ValueError:
    print('List does not contain value')

Output:

List does not contain value

Example Three

Code:

myList = [10, 9, 8, 7, 6, 5]

try: #Placing operation in try-except block
    i,j,k = myList
except ValueError:
    print('Too many values to unpack')

Output:

Too many values to unpack

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