How to differentiate between cases of ValueError

Question

Since too many python operations return ValueError, how can we differentiate between them?

Example: I expect an iterable to have a single element, and I want to get it

• a, = [1, 2]: ValueError: too many values to unpack
• a, = []: ValueError: too few values to unpack

How can I differentiate between those two cases?? eg

try:
    a, = lst
except ValueError as e:
    if e.too_many_values:
        do_this()
    else:
        do_that()

I realise that in this particular case I could find a work-around using length/indexing, but the point is similar cases come up often, and I want to know if there's a general approach. I also realise I could check the error message for if 'too few' in message but it seems a bit crude.

Answer 1

try:
    raise ValueError('my error')
except ValueError as e:

    # use str(), not repr(), see
    # https://stackoverflow.com/a/45532289/7919597

    x = getattr(e, 'message', str(e))

    if 'my error' in x:

        print('got my error')

(see also How to get exception message in Python properly)

But this might not be a clean solution after all.

The best thing would be to narrow the scope of your try block so that only one was possible. Or don't depend on exceptions to detect those error cases.