special-case Polars in .dt.to_string for Date

altair/utils/core.py

@@ -28,7 +28,7 @@
 
 import jsonschema
 import narwhals.stable.v1 as nw
-from narwhals.dependencies import is_pandas_dataframe
+from narwhals.dependencies import is_pandas_dataframe, get_polars
 from narwhals.typing import IntoDataFrame
 
 from altair.utils.schemapi import SchemaBase, Undefined
@@ -448,12 +448,14 @@ def sanitize_narwhals_dataframe(
     # See https://github.com/vega/altair/issues/1027 for why this is necessary.
     local_iso_fmt_string = "%Y-%m-%dT%H:%M:%S"
     for name, dtype in schema.items():
-        if dtype == nw.Date:
+        if dtype == nw.Date and nw.get_native_namespace(data) is get_polars():
             # Polars doesn't allow formatting `Date` with time directives.
             # The date -> datetime cast is extremely fast compared with `to_string`
             columns.append(
                 nw.col(name).cast(nw.Datetime).dt.to_string(local_iso_fmt_string)
             )
+        elif dtype == nw.Date:
+            columns.append(nw.col(name).dt.to_string(local_iso_fmt_string))
         elif dtype == nw.Datetime:
             columns.append(nw.col(name).dt.to_string(f"{local_iso_fmt_string}%.f"))
         elif dtype == nw.Duration:

tests/utils/test_utils.py

@@ -170,7 +170,7 @@ def test_sanitize_pyarrow_table_columns() -> None:
         "f": 0.0,
         "i": 0,
         "b": True,
-        "d": "2012-01-01T00:00:00.000000",
+        "d": "2012-01-01T00:00:00",
         "c": "a",
         "p": "2012-01-01T00:00:00.000000000",
     }

tests/vegalite/v5/test_api.py

@@ -1107,7 +1107,7 @@ def test_ibis_with_date_32():
     tbl = ibis.memtable(df)
     result = alt.Chart(tbl).mark_line().encode(x="a", y="b").to_dict()
     assert next(iter(result["datasets"].values())) == [
-        {"a": 1, "b": "2020-01-01T00:00:00.000000"},
-        {"a": 2, "b": "2020-01-02T00:00:00.000000"},
-        {"a": 3, "b": "2020-01-03T00:00:00.000000"},
+        {"a": 1, "b": "2020-01-01T00:00:00"},
+        {"a": 2, "b": "2020-01-02T00:00:00"},
+        {"a": 3, "b": "2020-01-03T00:00:00"},
     ]