Pytest text annotation for test with tuple of parameters

I'm looking for more elegant solution for this kind of problem:

def ids(x):
    if isinstance(x, int):
        return str(x)
    elif isinstance(x, str):
        return x[0]


@pytest.mark.parametrize("number, string",
                         [
                             (1, "One"),
                             (2, "Two"),
                             (3, "Three")
                         ],
                         ids=ids)
def test_stupid(number, string):
    assert 0 == 1

This code will produce test names: '1-O', '2-T', '3-T'.

The problem is that pytest is using same function for all arguments in tuple and I have to deal with ugly isinstance calls.

Is there a better way to solve this problem?

The only solution i've found is to use hook pytest_collection_modifyitems(session, config, items) It has access to parameters values, and can be used to change test names. But it relies on internal details of pytest Function class implementation and thus is not very robust.

I add pytest.mark with format string and converters for each parameter. All parameters optional

@pytest.mark.parametrize("number, string",
                     [
                         (1, "One"),
                         (2, "Two"),
                         (3, "Three")
                     ])
@pytest.mark.params_format('number={number} string={string}',
                           number=lambda x: str(x),
                           string=lambda x: x[0])
def test_stupid(number, string):

And then add to conftest.py hook implementation to use that mark parameters and add formatted string to test

def pytest_collection_modifyitems(session, config, items):
    for item in items:
        if not _get_marker(item, 'parametrize'):
            continue

        format_params_marker = _get_marker(item, 'params_format')
        if not format_params_marker:
            continue

        params = item.callspec.params.copy()
        param_formatters = format_params_marker.kwargs
        for param_name, presenter in param_formatters.iteritems():
            params[param_name] = presenter(params[param_name])

        params_names_ordered = [name for name in item.fixturenames if name in params]
        default_format = '-'.join(['{{{}}}'.format(param_name) for param_name in params_names_ordered])
        format_string = format_params_marker.args[0] if format_params_marker.args \
            else default_format
        item.name = '{func}[{params}]'.format(func=item.name.split('[')[0],
                                              params=format_string.format(**params))

def _get_marker(item, marker):
    return item.keywords._markers.get(marker, None)

The result will look something like this

test_stupid[number=1 string=O] PASSED
test_stupid[number=2 string=T] PASSED
test_stupid[number=3 string=T] PASSED