Proposal: apply case tags to all cases in class

[michele-riva]

Hi @smarie,

I found myself coming up with the following class decorator that allows applying a bunch of case tags to all the cases defined in a case class. It has the advantage to reduce code repetition if all cases in a class are to be tagged similarly or have a common set of tags.

Question 1: I did not find anything obvious to do the following already in pytest-cases. Did I overlook something?
Question 2: (if answer to previous is: "no it's not currently possible") Would it be something interesting for pytest-cases? If yes, I'm open to drafting a PR.

from pytest_cases import case as case_decorator
from pytest_cases.case_funcs import CASE_FIELD
from pytest_cases.case_funcs import is_case_class
from pytest_cases.case_funcs import is_case_function

def with_case_tags(*tags):
    """Attach `tags` to all cases defined in the decorated class."""
    def _decorator(cls):
        if is_case_function(cls):
            raise ValueError(
                'Cannot use with_case_tags on a case '
                'function. Use the @case decorator instead'
                )
        if not is_case_class(cls):
            raise ValueError('with_case_tags can only be applied to classes '
                             'defining a collection of cases')
        for case_name in dir(cls):
            case_ = getattr(cls, case_name)
            if not is_case_function(case_):  # Not a case
                continue
            try:
                case_info = getattr(case_, CASE_FIELD)
            except AttributeError:
                # Not explicitly decorated with @case. Do so now.
                case_ = case_decorator(case_)
                case_info = getattr(case_, CASE_FIELD)
            tags_to_add = tuple(t for t in tags if t not in case_info.tags)
            case_info.add_tags(tags_to_add)
        return cls
    return _decorator