API: raise on unsupported dtype instead of silently swapping

doc/source/whatsnew/v2.0.0.rst

@@ -99,6 +99,52 @@ notable_bug_fix2
 Backwards incompatible API changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. _whatsnew_200.api_breaking.unsupported_datetimelike_dtype_arg:
+
+Construction with datetime64 or timedelta64 dtype with unsupported resolution
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+In past versions, when constructing a :class:`Series` or :class:`DataFrame` and
+passing a "datetime64" or "timedelta64" dtype with unsupported resolution
+(i.e. anything other than "ns"), pandas would silently replace the given dtype
+with its nanosecond analogue:
+
+*Previous behavior*:
+
+.. code-block:: ipython
+
+   In [5]: pd.Series(["2016-01-01"], dtype="datetime64[s]")
+   Out[5]:
+   0   2016-01-01
+   dtype: datetime64[ns]
+
+   In [6]: pd.Series(["2016-01-01"], dtype="datetime64[D]")
+   Out[6]:
+   0   2016-01-01
+   dtype: datetime64[ns]
+
+In pandas 2.0 we support resolutions "s", "ms", "us", and "ns". When passing
+a supported dtype (e.g. "datetime64[s]"), the result now has exactly
+the requested dtype:
+
+*New behavior*:
+
+.. ipython:: python
+
+   In [5]: pd.Series(["2016-01-01"], dtype="datetime64[s]")
+   Out[5]:
+   0   2016-01-01
+   dtype: datetime64[s]
+
+With an un-supported dtype, pandas now raises instead of silently swapping in
+a supported dtype:
+
+*New behavior*:
+
+.. ipython:: python
+
+   In [6]: pd.Series(["2016-01-01"], dtype="datetime64[D]")
+   TypeError: dtype=datetime64[D] is not supported. Supported resolutions are 's', 'ms', 'us', and 'ns'
+
 .. _whatsnew_200.api_breaking.deps:
 
 Increased minimum versions for dependencies

pandas/core/dtypes/cast.py

@@ -31,10 +31,8 @@
     Timedelta,
     Timestamp,
     astype_overflowsafe,
-    get_supported_reso,
     get_unit_from_dtype,
     is_supported_unit,
-    npy_unit_to_abbrev,
 )
 from pandas._libs.tslibs.timedeltas import array_to_timedelta64
 from pandas._typing import (
@@ -1454,17 +1452,19 @@ def _ensure_nanosecond_dtype(dtype: DtypeObj) -> DtypeObj:
     """
     Convert dtypes with granularity less than nanosecond to nanosecond
 
-    >>> _ensure_nanosecond_dtype(np.dtype("M8[D]"))
-    dtype('<M8[s]')
-
     >>> _ensure_nanosecond_dtype(np.dtype("M8[us]"))
     dtype('<M8[us]')
 
+    >>> _ensure_nanosecond_dtype(np.dtype("M8[D]"))
+    Traceback (most recent call last):
+        ...
+    TypeError: dtype=datetime64[D] is not supported. Supported resolutions are 's', 'ms', 'us', and 'ns'
+
     >>> _ensure_nanosecond_dtype(np.dtype("m8[ps]"))
     Traceback (most recent call last):
         ...
-    TypeError: cannot convert timedeltalike to dtype [timedelta64[ps]]
-    """
+    TypeError: dtype=timedelta64[ps] is not supported. Supported resolutions are 's', 'ms', 'us', and 'ns'
+    """  # noqa:E501
     msg = (
         f"The '{dtype.name}' dtype has no unit. "
         f"Please pass in '{dtype.name}[ns]' instead."
@@ -1477,29 +1477,19 @@ def _ensure_nanosecond_dtype(dtype: DtypeObj) -> DtypeObj:
         # i.e. datetime64tz
         pass
 
-    elif dtype.kind == "M" and not is_supported_unit(get_unit_from_dtype(dtype)):
-        # pandas supports dtype whose granularity is less than [ns]
-        # e.g., [ps], [fs], [as]
-        if dtype <= np.dtype("M8[ns]"):
-            if dtype.name == "datetime64":
+    elif dtype.kind in ["m", "M"]:
+        reso = get_unit_from_dtype(dtype)
+        if not is_supported_unit(reso):
+            # pre-2.0 we would silently swap in nanos for lower-resolutions,
+            #  raise for above-nano resolutions
+            if dtype.name in ["datetime64", "timedelta64"]:
                 raise ValueError(msg)
-            reso = get_supported_reso(get_unit_from_dtype(dtype))
-            unit = npy_unit_to_abbrev(reso)
-            dtype = np.dtype(f"M8[{unit}]")
-        else:
-            raise TypeError(f"cannot convert datetimelike to dtype [{dtype}]")
-
-    elif dtype.kind == "m" and dtype != TD64NS_DTYPE:
-        # pandas supports dtype whose granularity is less than [ns]
-        # e.g., [ps], [fs], [as]
-        if dtype <= np.dtype("m8[ns]"):
-            if dtype.name == "timedelta64":
-                raise ValueError(msg)
-            reso = get_supported_reso(get_unit_from_dtype(dtype))
-            unit = npy_unit_to_abbrev(reso)
-            dtype = np.dtype(f"m8[{unit}]")
-        else:
-            raise TypeError(f"cannot convert timedeltalike to dtype [{dtype}]")
+            # TODO: ValueError or TypeError? existing test
+            #  test_constructor_generic_timestamp_bad_frequency expects TypeError
+            raise TypeError(
+                f"dtype={dtype} is not supported. Supported resolutions are 's', "
+                "'ms', 'us', and 'ns'"
+            )
     return dtype
 
 

pandas/tests/series/test_constructors.py

@@ -1669,19 +1669,23 @@ def test_constructor_generic_timestamp_no_frequency(self, dtype, request):
         with pytest.raises(ValueError, match=msg):
             Series([], dtype=dtype)
 
-    @pytest.mark.parametrize(
-        "dtype,msg",
-        [
-            ("m8[ps]", "cannot convert timedeltalike"),
-            ("M8[ps]", "cannot convert datetimelike"),
-        ],
-    )
-    def test_constructor_generic_timestamp_bad_frequency(self, dtype, msg):
+    @pytest.mark.parametrize("unit", ["ps", "as", "fs", "Y", "M", "W", "D", "h", "m"])
+    @pytest.mark.parametrize("kind", ["m", "M"])
+    def test_constructor_generic_timestamp_bad_frequency(self, kind, unit):
         # see gh-15524, gh-15987
+        # as of 2.0 we raise on any non-supported unit rather than silently
+        #  cast to nanos; previously we only raised for frequencies higher
+        #  than ns
+        dtype = f"{kind}8[{unit}]"
 
+        msg = "dtype=.* is not supported. Supported resolutions are"
         with pytest.raises(TypeError, match=msg):
             Series([], dtype=dtype)
 
+        with pytest.raises(TypeError, match=msg):
+            # pre-2.0 the DataFrame cast raised but the Series case did not
+            DataFrame([[0]], dtype=dtype)
+
     @pytest.mark.parametrize("dtype", [None, "uint8", "category"])
     def test_constructor_range_dtype(self, dtype):
         # GH 16804