Why if Is Not Enough: Understanding try/except in Python

While I was writing a tip calculator in Python, you can check my GitHub for the full code. I realized that even though I used an if condition, errors were still happening. The reason is that the if condition runs after the type conversion, but the error happens during the conversion itself. 

        def get_bill_amount(prompt: str) -> float:
            while True:
            value = input(prompt).strip()
            try:
                amount = float(value)
                if amount > 0:
                    return amount
                print("Amount must be greater than 0.")
            except ValueError:
                print("Please enter a valid number.")

Expected user input: a number greater than 0

Type mismatch: when the user enters a string like abc

Error: the program crashes with ValueError: could not convert string to float

The key point is that float(value) is a risky operation. If the conversion fails, Python throws an error before the if condition is even checked.

Using value.isdigit() may look safe, but it fails for valid inputs like 12.5, -3, or even 10 with spaces. This is why try/except exists.

  • Rule of thumb:

    • Always use if to validate rules, and try/except to protect your program from crashing.