ValueError

ValueError Occurs When

• Calling a function with an argument that has the right type but an inappropriate value
• Converting strings to numbers and the string doesn’t represent a valid number

ValueError Can Be Used When

• Validating function arguments to ensure they’re in acceptable ranges or sets
• Handling user input that needs specific formatting or value constraints
• Enforcing value restrictions in custom data types or classes

ValueError Examples

Here’s an example where Python raises a ValueError when you try converting a string that doesn’t represent a valid number into an integer:

>>> int("one")
Traceback (most recent call last):
    ...
ValueError: invalid literal for int() with base 10: 'one'

You can handle ValueError with a try … except block to show a user-friendly message or choose an alternative action:

>>> user_input = "one"
>>> try:
...     number = int(user_input)
... except ValueError:
...     print(f"Could not convert '{user_input}' to an integer.")
...
Could not convert 'one' to an integer.

If you’re writing a function that demands certain value constraints, you can raise ValueError to enforce them:

>>> def set_age(age):
...     if age < 0:
...         raise ValueError("Age can't be negative")
...     print(f"Age is {age} years")

>>> set_age(-5)
Traceback (most recent call last):
    ...
ValueError: Age can't be negative

Related Resources

Tutorial

Python's Built-in Exceptions: A Walkthrough With Examples

In this tutorial, you'll get to know some of the most commonly used built-in exceptions in Python. You'll learn when these exceptions can appear in your code and how to handle them. Finally, you'll learn how to raise some of these exceptions in your code.

intermediate python

For additional information on related topics, take a look at the following resources:

• Python's raise: Effectively Raising Exceptions in Your Code (Tutorial)
• Strings and Character Data in Python (Tutorial)
• Numbers in Python (Tutorial)
• Python's Built-in Exceptions: A Walkthrough With Examples (Quiz)
• Using raise for Effective Exceptions (Course)
• Python's raise: Effectively Raising Exceptions in Your Code (Quiz)
• Strings and Character Data in Python (Course)
• Python Strings and Character Data (Quiz)