API: is_string_dtype is not strict

[jreback]

#15533 (comment)

pandas.types.common.is_string_dtype is not strict, just checking if its a fixed-width numpy string/unicode type or object dtype. It could be made strict via something like this.

def is_string_dtype(arr_or_dtype):
    dtype = _get_dtype(arr_or_dtype)
    if isinstance(arr_or_dtype, np.ndarray):
        if not lib.infer_dtype(arr_or_dtype) in ['string', 'unicode']:
            return False

    return dtype.kind in ('O', 'S', 'U') and not is_period_dtype(dtype)

this would need performance checking to see if its a problem. Further this then changes the API a tiny bit (as we allow an object OR a dtype to be passed in). Which is probably ok as long as its documented a bit.

[jreback]

xref @ResidentMario

[ResidentMario]

There definitely needs to be a strict implementation somewhere in types, I have a working-ish solution for #15533 that depends on this.

Maybe also consider leaving this method as-is and throwing in a new is_string_dtype_strict method with this signature, if performance is a problem.

How to test performance? Will timeit do, or need to add something to asv_bench?

[jreback]

asv is the place for things like this.

[jreback]

@ResidentMario you shouldn't need this fyi