partial fix for python datetime conversions (fractional seconds curre…

…ntly get dropped) (#1857)

py-polars/polars/internals/series.py

@@ -3440,8 +3440,7 @@ def to_python_datetime(self) -> Series:
         """
         Go from Date/Datetime to python DateTime objects
         """
-
-        return (self.timestamp() // 1000).apply(
+        return (self.timestamp() / 1000).apply(
             lambda ts: datetime.utcfromtimestamp(ts), Object
         )
 
@@ -3510,7 +3509,7 @@ def _to_python_datetime(
         return datetime.utcfromtimestamp(value * 3600 * 24).date()
     elif dtype == Datetime:
         # ms to seconds
-        return datetime.utcfromtimestamp(value // 1000)
+        return datetime.utcfromtimestamp(value / 1000)
     else:
         raise NotImplementedError
 

py-polars/tests/test_datelike.py

@@ -151,7 +151,8 @@ def test_to_python_datetime() -> None:
 
 
 def test_datetime_consistency() -> None:
-    dt = datetime(2021, 1, 1)
+    # dt = datetime(2021, 1, 1, 10, 30, 45, 123456)
+    dt = datetime(2021, 1, 1, 10, 30, 45, 123000)
     df = pl.DataFrame({"date": [dt]})
     assert df["date"].dt[0] == dt
     assert df.select(pl.lit(dt))["literal"].dt[0] == dt

py-polars/tests/test_io.py

@@ -101,7 +101,7 @@ def test_parquet_chunks() -> None:
 
 def test_parquet_datetime() -> None:
     """
-    This failed because parquet writers cast datetimeto Date
+    This failed because parquet writers cast datetime to Date
     """
     f = io.BytesIO()
     data = {