fix[python]: Series init issue with unit/precision (from python datet…

…ime) (#4921)

py-polars/polars/internals/construction.py

@@ -230,7 +230,14 @@ def sequence_to_pyseries(
 
     else:
         if python_dtype is None:
-            python_dtype = float if (value is None) else type(value)
+            if value is None:
+                # generic default dtype
+                python_dtype = float
+            else:
+                python_dtype = type(value)
+                if datetime == python_dtype:
+                    # note: python-native datetimes have microsecond precision
+                    temporal_unit = "us"
 
         # temporal branch
         if python_dtype in py_temporal_types:

py-polars/tests/unit/test_datelike.py

@@ -244,6 +244,22 @@ def test_datetime_consistency() -> None:
         )
     ]
 
+    test_data = [
+        datetime(2000, 1, 1, 1, 1, 1, 555555),
+        datetime(2514, 5, 30, 1, 53, 4, 986754),
+        datetime(3099, 12, 31, 23, 59, 59, 123456),
+        datetime(9999, 12, 31, 23, 59, 59, 999999),
+    ]
+    ddf = pl.DataFrame({"dtm": test_data}).with_column(
+        pl.col("dtm").dt.nanosecond().alias("ns")
+    )
+    assert ddf.rows() == [
+        (test_data[0], 555555000),
+        (test_data[1], 986754000),
+        (test_data[2], 123456000),
+        (test_data[3], 999999000),
+    ]
+
 
 def test_timezone() -> None:
     ts = pa.timestamp("s")