PERF/DISC: restrict inputs to is_foo_dtype

[jbrockmendel]

xref #33364

A lot of the overhead in is_foo_dtype (which we call a lot) comes from checking for dtype objects, arrays, and strings. We could tighten these to be e.g.

def is_extension_arrray_dtype(obj):
     return isinstance(obj, ExtensionDtype)

The trouble with this is some of these are user-facing, so this would require a deprecation cycle, and in the interim the warnings call would actually hurt performance.

[TomAugspurger]

The trouble with this is some of these are user-facing, so this would require a deprecation cycle, and in the interim the warnings call would actually hurt performance.

It'll be a bit of work, but we can define

is_extension_array_dtype(obj):
    # flexible stuff...
    # deprecation checks...
    return _is_extension_array_dtype(dtype)

And internally we use the strict _is_extension_array_dtype. That's probably worth doing regardless of whether we deprecate the flexible stuff, and only accept dtype instances.

[jbrockmendel]

And internally we use the strict _is_extension_array_dtype

Makes sense. Should keep the names public (no underscore, not in the public API), maybe some abbreviation is_ea_dtype, is_dt64_dtype, ...

[jorisvandenbossche]

Big +1 to deprecate passing values instead of dtypes.

I am not sure it needs to hurt performance to include the deprecation? (as long as internally we change all occurrences to call this with a dtype of course).

Right now, we eg have have this function to get the dtype:

pandas/pandas/core/dtypes/common.py

Lines 1594 to 1607 in 70086c5

	if arr_or_dtype is None:
	raise TypeError("Cannot deduce dtype from null object")
	# fastpath
	elif isinstance(arr_or_dtype, np.dtype):
	return arr_or_dtype
	elif isinstance(arr_or_dtype, type):
	return np.dtype(arr_or_dtype)
	# if we have an array-like
	elif hasattr(arr_or_dtype, "dtype"):
	arr_or_dtype = arr_or_dtype.dtype
	return pandas_dtype(arr_or_dtype)

So there is a check to see if the object has a "dtype" attribute. Adding a deprecation warning inside that if block shouldn't impact performance when passing dtypes to it?