perf: to_datetime() now avoids caching inputs unless data is inspected to infer format

bigframes/core/tools/datetimes.py

@@ -52,7 +52,7 @@ def to_datetime(
             f"to datetime is not implemented. {constants.FEEDBACK_LINK}"
         )
 
-    arg = bigframes.series.Series(arg)._cached()
+    arg = bigframes.series.Series(arg)
 
     if format and unit and arg.dtype in (bigframes.dtypes.INT_DTYPE, bigframes.dtypes.FLOAT_DTYPE):  # type: ignore
         raise ValueError("cannot specify both format and unit")
@@ -74,6 +74,11 @@ def to_datetime(
             )
 
         assert unit is None
+
+        # The following operations evaluate individual values to infer a format,
+        # so cache if needed.
+        arg = arg._cached(force=False)
+
         as_datetime = arg._apply_unary_op(  # type: ignore
             ops.ToDatetimeOp(
                 format=format,

tests/unit/core/tools/__init__.py

@@ -0,0 +1,13 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.

tests/unit/core/tools/test_datetimes.py

@@ -0,0 +1,43 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import cast
+from unittest import mock
+
+import bigframes.core.tools.datetimes
+import bigframes.dtypes
+import bigframes.pandas
+import bigframes.testing.mocks
+
+
+def test_to_datetime_with_series_and_format_doesnt_cache(monkeypatch):
+    df = bigframes.testing.mocks.create_dataframe(monkeypatch)
+    series = mock.Mock(spec=bigframes.pandas.Series, wraps=df["col"])
+    dt_series = cast(
+        bigframes.pandas.Series,
+        bigframes.core.tools.datetimes.to_datetime(series, format="%Y%m%d"),
+    )
+    series._cached.assert_not_called()
+    assert dt_series.dtype == bigframes.dtypes.DATETIME_DTYPE
+
+
+def test_to_datetime_with_series_and_format_utc_doesnt_cache(monkeypatch):
+    df = bigframes.testing.mocks.create_dataframe(monkeypatch)
+    series = mock.Mock(spec=bigframes.pandas.Series, wraps=df["col"])
+    dt_series = cast(
+        bigframes.pandas.Series,
+        bigframes.core.tools.datetimes.to_datetime(series, format="%Y%m%d", utc=True),
+    )
+    series._cached.assert_not_called()
+    assert dt_series.dtype == bigframes.dtypes.TIMESTAMP_DTYPE