ValueError is a built-in exception class in Python that is raised when an operation or function receives an argument that has the right type but an inappropriate value. For example, trying to convert a string that is not a valid number to an integer will raise a ValueError.

Example of ValueError

>>> int('hello')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'hello'

How to catch ValueError

To handle ValueError, we can use a try-except block and catch the exception using the except keyword. We can also access the error message using the .args attribute of the exception object. For example:

try:
    num = int(input('Enter a number: '))
    print(f'You entered {num}')
except ValueError as e:
    print(f'Invalid input: {e.args[0]}')

How to raise ValueError

Sometimes, we may want to raise a ValueError ourselves if we detect that the input or argument is not valid for our function or operation. To do this, we can use the raise keyword and pass a custom error message as an argument to the ValueError class. For example:

def factorial(n):
    if n < 0:
        raise ValueError('n must be non-negative')
    if n == 0 or n == 1:
        return 1
    return n * factorial(n - 1)

Example of raising ValueError

>>> factorial(-1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in factorial
ValueError: n must be non-negative

Most Common Value Errors In Python

• Valueerror: math domain error
• Valueerror: invalid literal for int with base 10
• Valueerror: i/o operation on closed file

Conclusion

ValueError is a common exception that occurs when the value of an argument or input is not suitable for the function or operation. We can handle it using try-except blocks and raise it using the raise keyword. We can also access the error message using the .args attribute of the exception object.