What is ValueError in Python?

A ValueError in Python occurs when a function receives an argument with the right type but an invalid value. It is a common exception that beginners encounter.

Common Causes of ValueError

Several situations can trigger a ValueError. Understanding these helps in debugging.

One common cause is converting a string to an integer when the string is not numeric. For example:

 
# Trying to convert a non-numeric string to an integer
num = int("abc")

ValueError: invalid literal for int() with base 10: 'abc'

Another cause is passing an out-of-range value to a function. For example, using a negative value for math.sqrt().

How to Fix ValueError

To fix a ValueError, validate inputs before processing. Use try-except blocks to handle errors gracefully.

Here’s an example of handling invalid integer conversion:

 
try:
    num = int("123abc")
except ValueError:
    print("Invalid number. Please enter a valid integer.")

Invalid number. Please enter a valid integer.

For more details, check our guide on Understanding ValueError in Python.

Examples of ValueError in Python

Let’s look at more examples to understand ValueError better.

Example 1: Invalid base conversion with int().

 
# Trying to convert a string with invalid base
num = int("101", base=2)  # Valid binary
invalid_num = int("102", base=2)  # Invalid binary

ValueError: invalid literal for int() with base 2: '102'

Example 2: Unpacking mismatched values.

 
# Unpacking more values than variables
a, b = [1, 2, 3]

ValueError: too many values to unpack (expected 2)

Best Practices to Avoid ValueError

Follow these tips to minimize ValueError occurrences in your code.

Always validate user inputs. Use try-except blocks for risky operations. Check function documentation for valid value ranges.

For example, ensure a list has enough items before unpacking:

 
values = [1, 2]
if len(values) == 2:
    a, b = values
else:
    print("List must have exactly 2 items.")

Learn more about handling errors in our Understanding ValueError in Python guide.

Conclusion

A ValueError occurs when a function gets an invalid value. Common causes include type conversion and out-of-range values.

Use input validation and error handling to manage it. This ensures your code runs smoothly even with unexpected inputs.

For more Python tips, explore our resources on Understanding ValueError in Python.