DEPR: disallow unit-less dt64 dtype in astype

doc/source/whatsnew/v2.0.0.rst

@@ -175,6 +175,7 @@ Removal of prior version deprecations/changes
 - Removed deprecated :meth:`DatetimeIndex.to_perioddelta`, Use ``dtindex - dtindex.to_period(freq).to_timestamp()`` instead (:issue:`34853`)
 - Enforced deprecation disallowing passing a timezone-aware :class:`Timestamp` and ``dtype="datetime64[ns]"`` to :class:`Series` or :class:`DataFrame` constructors (:issue:`41555`)
 - Enforced deprecation disallowing passing a sequence of timezone-aware values and ``dtype="datetime64[ns]"`` to to :class:`Series` or :class:`DataFrame` constructors (:issue:`41555`)
+- Enforced deprecation disallowing unit-less "datetime64" dtype in :meth:`Series.astype` and :meth:`DataFrame.astype` (:issue:`47844`)
 - Enforced deprecation disallowing using ``.astype`` to convert a ``datetime64[ns]`` :class:`Series`, :class:`DataFrame`, or :class:`DatetimeIndex` to timezone-aware dtype, use ``obj.tz_localize`` or ``ser.dt.tz_localize`` instead (:issue:`39258`)
 - Enforced deprecation disallowing using ``.astype`` to convert a timezone-aware :class:`Series`, :class:`DataFrame`, or :class:`DatetimeIndex` to timezone-naive ``datetime64[ns]`` dtype, use ``obj.tz_localize(None)`` or ``obj.tz_convert("UTC").tz_localize(None)`` instead (:issue:`39258`)
 - Removed Date parser functions :func:`~pandas.io.date_converters.parse_date_time`,

pandas/core/arrays/datetimes.py

@@ -683,15 +683,10 @@ def astype(self, dtype, copy: bool = True):
             and dtype != self.dtype
             and is_unitless(dtype)
         ):
-            # TODO(2.0): just fall through to dtl.DatetimeLikeArrayMixin.astype
-            warnings.warn(
-                "Passing unit-less datetime64 dtype to .astype is deprecated "
-                "and will raise in a future version. Pass 'datetime64[ns]' instead",
-                FutureWarning,
-                stacklevel=find_stack_level(),
+            raise TypeError(
+                "Casting to unit-less dtype 'datetime64' is not supported. "
+                "Pass e.g. 'datetime64[ns]' instead."
             )
-            # unit conversion e.g. datetime64[s]
-            return self._ndarray.astype(dtype)
 
         elif is_period_dtype(dtype):
             return self.to_period(freq=dtype.freq)

pandas/tests/series/methods/test_astype.py

@@ -30,18 +30,20 @@
 
 
 class TestAstypeAPI:
-    def test_astype_unitless_dt64_deprecated(self):
+    def test_astype_unitless_dt64_raises(self):
         # GH#47844
         ser = Series(["1970-01-01", "1970-01-01", "1970-01-01"], dtype="datetime64[ns]")
+        df = ser.to_frame()
 
-        msg = "Passing unit-less datetime64 dtype to .astype is deprecated and "
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            res = ser.astype(np.datetime64)
-        tm.assert_series_equal(ser, res)
-
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            res = ser.astype("datetime64")
-        tm.assert_series_equal(ser, res)
+        msg = "Casting to unit-less dtype 'datetime64' is not supported"
+        with pytest.raises(TypeError, match=msg):
+            ser.astype(np.datetime64)
+        with pytest.raises(TypeError, match=msg):
+            df.astype(np.datetime64)
+        with pytest.raises(TypeError, match=msg):
+            ser.astype("datetime64")
+        with pytest.raises(TypeError, match=msg):
+            df.astype("datetime64")
 
     def test_arg_for_errors_in_astype(self):
         # see GH#14878