DEPR: passing a dtype alias to DatetimeTZDtype(unit) (pandas-dev#29927)

Author: jbrockmendel

doc/source/whatsnew/v1.0.0.rst

@@ -427,6 +427,7 @@ or ``matplotlib.Axes.plot``. See :ref:`plotting.formatters` for more.
 - Removed the previously deprecated :meth:`MultiIndex.set_labels`, use :meth:`MultiIndex.set_codes` instead (:issue:`23752`)
 - Removed the previously deprecated "labels" keyword from :meth:`MultiIndex.set_codes`, :meth:`MultiIndex.copy`, :meth:`MultiIndex.drop`, use "codes" instead (:issue:`23752`)
 - Removed support for legacy HDF5 formats (:issue:`29787`)
+- Passing a dtype alias (e.g. 'datetime64[ns, UTC]') to :class:`DatetimeTZDtype` is no longer allowed, use :meth:`DatetimeTZDtype.construct_from_string` instead (:issue:`23990`)
 - :func:`read_excel` removed support for "skip_footer" argument, use "skipfooter" instead (:issue:`18836`)
 - :func:`read_excel` no longer allows an integer value for the parameter ``usecols``, instead pass a list of integers from 0 to ``usecols`` inclusive (:issue:`23635`)
 - :meth:`DataFrame.to_records` no longer supports the argument "convert_datetime64" (:issue:`18902`)

pandas/core/dtypes/dtypes.py

@@ -676,7 +676,7 @@ def __init__(self, unit="ns", tz=None):
                     "to DatetimeTZDtype is deprecated. Use "
                     "'DatetimeTZDtype.construct_from_string()' instead."
                 )
-                warnings.warn(msg.format(tz=tz), FutureWarning, stacklevel=2)
+                raise ValueError(msg)
             else:
                 raise ValueError("DatetimeTZDtype only supports ns units")
 

pandas/tests/dtypes/test_dtypes.py

@@ -187,7 +187,7 @@ def create(self):
 
     def test_alias_to_unit_raises(self):
         # 23990
-        with tm.assert_produces_warning(FutureWarning):
+        with pytest.raises(ValueError, match="Passing a dtype alias"):
             DatetimeTZDtype("datetime64[ns, US/Central]")
 
     def test_alias_to_unit_bad_alias_raises(self):