Introduction to Error Handling

When Conversion Fails

When a program asks for input with input(), Python receives a string. If the program needs to use that response as a number, it can pass the string to int() or float(). The conversion can fail when the characters do not describe a value of the requested type. For example, passing the string "hello" to int() causes Python to raise a ValueError.

Tracing the Input

The conversion function examines the characters in the string. For integer conversion, a string can contain only digits and may optionally begin with a plus or minus sign. If the characters are valid, int() returns a new integer. If any character is invalid, conversion stops and Python raises a ValueError.

The Try and Except Path

A try/except block gives the program two possible paths. The try block contains the code that might raise an error, such as converting user input with int(). If that code succeeds, Python continues after the try/except structure. If a ValueError occurs, Python moves to the except ValueError block and runs its indented handling code instead of allowing the failure to crash the program.

try:
    number = int(user_input)
except ValueError:
    print("Please enter a whole number.")

The indented statement inside try is the operation that might fail. The phrase except ValueError identifies the error this handler is prepared to catch. The indented statement inside except runs only when the conversion raises that error.

A Complete User Input Example

response = input("Enter a whole number: ")

try:
    number = int(response)
    print("You entered", number)
except ValueError:
    print("That input is not a valid whole number.")

If the user enters 42, the conversion succeeds and the program prints:
You entered 42

If the user enters hello, the conversion raises a ValueError and the program prints:
That input is not a valid whole number.

Graceful Recovery

User input is unpredictable, so conversions of user input should be protected with try/except. A handled ValueError lets the program respond deliberately, such as by asking the user to try again or providing a sensible default behavior. The important change is in the program's response: the failure is handled in the except block rather than being left to crash the program.

Mistakes to Avoid

• Assuming that input() returns a number when the user types digits.

  If the program needs a number, it must convert the string with int() or float().

  Fix: Pass the input string to the appropriate conversion function.

• Converting user input without handling ValueError.

  An input such as hello cannot be converted to an integer, so the unhandled error can crash the program.

  Fix: Place the conversion in a try block and catch ValueError in an except block.

• Treating every string as valid integer input.

  Integer conversion accepts digits and an optional leading plus or minus sign, not arbitrary letters.

  Fix: Handle invalid input with except ValueError and explain the expected format.

• Giving no useful guidance after a failed conversion.

  The user may not understand how to fix the input.

  Fix: Provide a clear error message describing the expected input.

Practice the Control Flow

Key Takeaways

1. input() provides a string, even when the user types digits.
2. int() can convert strings containing digits and an optional leading plus or minus sign; invalid content can cause a ValueError.
3. A ValueError is raised at runtime when the requested type conversion fails.
4. The try block contains code that might fail, and except ValueError contains the response to that failure.
5. Handling user-input conversions with try/except and clear messages prevents an unpleasant unhandled crash.