fix[rust]: don't unnecessarily truncate datetimes on string casts (#4625

)
pola-rs · Aug 30, 2022 · c5083a2 · c5083a2
1 parent 1deac61
commit c5083a2
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Showing 2 changed files with 36 additions and 2 deletions.
diff --git a/polars/polars-core/src/series/implementations/datetime.rs b/polars/polars-core/src/series/implementations/datetime.rs
@@ -378,8 +378,16 @@ impl SeriesTrait for SeriesWrap<DatetimeChunked> {
     }
 
     fn cast(&self, data_type: &DataType) -> Result<Series> {
-        match data_type {
-            DataType::Utf8 => Ok(self.0.strftime("%F %T").into_series()),
+        match (data_type, self.0.time_unit()) {
+            (DataType::Utf8, TimeUnit::Milliseconds) => {
+                Ok(self.0.strftime("%F %T%.3f").into_series())
+            }
+            (DataType::Utf8, TimeUnit::Microseconds) => {
+                Ok(self.0.strftime("%F %T%.6f").into_series())
+            }
+            (DataType::Utf8, TimeUnit::Nanoseconds) => {
+                Ok(self.0.strftime("%F %T%.9f").into_series())
+            }
             _ => self.0.cast(data_type),
         }
     }

diff --git a/py-polars/tests/test_datelike.py b/py-polars/tests/test_datelike.py
@@ -1426,3 +1426,29 @@ def test_date_timedelta() -> None:
         "date_plus_one": [date(2001, 1, 2), date(2001, 1, 3), date(2001, 1, 4)],
         "date_min_one": [date(2000, 12, 31), date(2001, 1, 1), date(2001, 1, 2)],
     }
+
+
+def test_datetime_string_casts() -> None:
+    df = pl.DataFrame(
+        {
+            "x": [1661855445123],
+            "y": [1661855445123456],
+            "z": [1661855445123456789],
+        },
+        columns=[
+            ("x", pl.Datetime("ms")),
+            ("y", pl.Datetime("us")),
+            ("z", pl.Datetime("ns")),
+        ],
+    )
+    assert df.select(
+        [pl.col("x").dt.strftime("%F %T").alias("w")]
+        + [pl.col(d).cast(str) for d in df.columns]
+    ).rows() == [
+        (
+            "2022-08-30 10:30:45",
+            "2022-08-30 10:30:45.123",
+            "2022-08-30 10:30:45.123456",
+            "2022-08-30 10:30:45.123456789",
+        )
+    ]