Exception handling: what exception to raise

Question

I am writing a python function that accepts certain parameters. I would like to make sure one of the parameter's value is a string of specific custom format. If it doesn't match the format I would like to raise an exception. Is it appropriate to raise one of the built-in exception and if yes which one?

I looked here: https://docs.python.org/3/library/exceptions.html# but couldn't pin down to a specific one.

Answer 1

As long as it provides a detailed and explicit error message, you can use a built-in one - ValueError, for example, looks logical here.

Another option would be to create a custom one:

class InvalidFormatError(ValueError):
    pass

There are relevant threads on SO that, I hope, would help you to decide which option to choose:

• When to use custom exceptions vs. existing exceptions vs. generic exceptions
• Is it OK to raise a built-in exception, but with a different message, in Python?

Answer 2

What you describe sounds like a ValueError:

Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as IndexError.

Answer 3

I would use ValueError:

Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as IndexError.

That it is raised by built in functions doesn't mean you can't raise it too.