BUG: DatetimeArray.unit when constructed from a non-nano ndarray

doc/source/whatsnew/v2.1.0.rst

@@ -219,6 +219,7 @@ Datetimelike
 - Bug in :meth:`Timestamp.round` with values close to the implementation bounds returning incorrect results instead of raising ``OutOfBoundsDatetime`` (:issue:`51494`)
 - :meth:`DatetimeIndex.map` with ``na_action="ignore"`` now works as expected. (:issue:`51644`)
 - Bug in :meth:`arrays.DatetimeArray.map` and :meth:`DatetimeIndex.map`, where the supplied callable operated array-wise instead of element-wise (:issue:`51977`)
+- Bug in :class:`arrays.DatetimeArray` constructor returning an incorrect unit when passed a non-nanosecond numpy datetime array (:issue:`52555`)
 -
 
 Timedelta

pandas/core/arrays/datetimelike.py

@@ -1885,7 +1885,10 @@ def __init__(
             values = values._ndarray
 
         elif dtype is None:
-            dtype = self._default_dtype
+            if isinstance(values, np.ndarray) and values.dtype.kind in "Mm":
+                dtype = values.dtype
+            else:
+                dtype = self._default_dtype
 
         if not isinstance(values, np.ndarray):
             raise ValueError(

pandas/tests/arrays/datetimes/test_constructors.py

@@ -127,6 +127,13 @@ def test_copy(self):
         arr = DatetimeArray(data, copy=True)
         assert arr._ndarray is not data
 
+    @pytest.mark.parametrize("unit", ["s", "ms", "us", "ns"])
+    def test_numpy_datetime_unit(self, unit):
+        data = np.array([1, 2, 3], dtype=f"M8[{unit}]")
+        arr = DatetimeArray(data)
+        assert arr.unit == unit
+        assert arr[0].unit == unit
+
 
 class TestSequenceToDT64NS:
     def test_tz_dtype_mismatch_raises(self):