fix tests

Author: TrevorBergeron

bigframes/core/compile/ibis_compiler/scalar_op_registry.py

@@ -982,7 +982,9 @@ def isin_op_impl(x: ibis_types.Value, op: ops.IsInOp):
 
 @scalar_op_compiler.register_unary_op(ops.ToDatetimeOp, pass_op=True)
 def to_datetime_op_impl(x: ibis_types.Value, op: ops.ToDatetimeOp):
-    if x.type() in (ibis_dtypes.str, ibis_dtypes.Timestamp("UTC")):  # type: ignore
+    if x.type() == ibis_dtypes.Timestamp(None):  # type: ignore
+        return x  # already a timestamp, no-op
+    elif x.type() in (ibis_dtypes.str, ibis_dtypes.Timestamp("UTC")):  # type: ignore
         return x.try_cast(ibis_dtypes.Timestamp(None))  # type: ignore
     else:
         # Numerical inputs.

tests/system/small/operations/test_datetimes.py

@@ -342,19 +342,20 @@ def test_dt_tz_localize(scalars_dfs, col_name, tz):
     assert_series_equal(bf_result.to_pandas(), pd_result, check_index_type=False)
 
 
-@pytest.mark.parametrize(
-    ("col_name", "tz"),
-    [
-        ("timestamp_col", "UTC"),
-        ("datetime_col", "US/Eastern"),
-    ],
-)
-def test_dt_tz_localize_invalid_inputs(scalars_dfs, col_name, tz):
+def test_dt_tz_localize_already_localized(scalars_dfs):
+    pytest.importorskip("pandas", minversion="2.0.0")
+    scalars_df, _ = scalars_dfs
+
+    with pytest.raises(TypeError):
+        scalars_df["timestamp_col"].dt.tz_localize("UTC")
+
+
+def test_dt_tz_localize_invalid_timezone(scalars_dfs):
     pytest.importorskip("pandas", minversion="2.0.0")
     scalars_df, _ = scalars_dfs
 
     with pytest.raises(ValueError):
-        scalars_df[col_name].dt.tz_localize(tz)
+        scalars_df["datetime_col"].dt.tz_localize("US/Eastern")
 
 
 @pytest.mark.parametrize(