Python What Happens When a Number Cannot Be Converted?

This guide explains What Happens When a Number Cannot Be Converted? through a focused example and practical reasoning. No prior knowledge of this specific operation is required.

Understanding the Concept

Numeric conversion can raise ValueError when the text is not a valid representation of the requested number. It can also raise TypeError for unsupported input types. Handle predictable user mistakes without hiding unrelated failures.

When You Would Use This

A price entry should explain invalid input rather than terminating with a traceback.

Step-by-Step Example

price_text = "12.50 USD"
try:
    amount = float(price_text)
except ValueError:
    print("Enter a number only, such as 12.50.")
else:
    print(f"Price: {amount:.2f}")

How the Example Works

The text includes USD, so float() raises ValueError. The program catches that specific error and prints a useful instruction. Remove USD and the calculation succeeds.

Example Output

Enter a number only, such as 12.50.

Try It Yourself

Modify the example to accept leading and trailing spaces, then test empty text.

Common Beginner Mistakes

Catching every exception and pretending all failures mean bad user input.
Assuming int("8.0") works because 8.0 is mathematically a whole number.
Using a bare except that also hides programming errors and interrupts.

Key Takeaways

Numeric conversion can raise ValueError when the text is not a valid representation of the requested number.
The text includes USD, so float() raises ValueError.
Avoid this mistake: Catching every exception and pretending all failures mean bad user input.