How to Use try except ValueError in Python to Parse Numbers

Convert strings to integers safely with try/except ValueError and TypeError, returning a value-or-error tuple.

Easy Python 3.9+ Aug 9, 2026 Errors & debugging 13 views 0 copies

try-except valueerror error-handling parsing beginners

Python code

20 lines

Python 3.9+

def parse_number(text):
    """Safely convert a string to an integer, handling errors gracefully."""
    try:
        value = int(text)
        return value, None
    except ValueError as error:
        return None, f"Conversion failed: {error}"
    except TypeError as error:
        return None, f"Wrong type provided: {error}"


if __name__ == "__main__":
    test_inputs = ["42", "hello", "7.5", None]

    for item in test_inputs:
        result, error = parse_number(item)
        if error:
            print(f"Input '{item}' -> ERROR: {error}")
        else:
            print(f"Input '{item}' -> SUCCESS: {result}")

Output

stdout

Input '42' -> SUCCESS: 42
Input 'hello' -> ERROR: Conversion failed: invalid literal for int() with base 10: 'hello'
Input '7.5' -> ERROR: Conversion failed: invalid literal for int() with base 10: '7.5'
Input 'None' -> ERROR: Wrong type provided: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

How it works

The try block attempts int(text) and signals success by returning (value, None). On a ValueError (e.g., non-numeric string) or TypeError (e.g., None), the handler returns (None, error_message) instead of crashing. The tuple pattern lets callers check the second element to know if conversion failed, avoiding exceptions at the call site. Because int() raises ValueError for most bad strings and TypeError for wrong types, catching both covers common failure modes. This design keeps the function pure and easy to test, with no side effects or printed output inside.

Common mistakes

Catching `Exception` too broadly and hiding real bugs
Forgetting `TypeError` when `None` or a float-like object is passed
Returning `None` for both error and value, confusing the caller
Swallowing the error without saving the message for debugging

Variations

Use `try` with a single `except (ValueError, TypeError)` to simplify if you don't need to distinguish error types.
Use `if isinstance(text, str):` checks before converting to avoid exceptions entirely.

Real-world use cases

Parsing user input from a CLI or web form where invalid numbers must show a friendly error.
Validating configuration file values before using them in a data pipeline.
Handling messy CSV or API text fields that may contain non-numeric strings.

How to Use try except ValueError in Python to Parse Numbers

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Real-world use cases

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