address comments

Author: Linchin

google/cloud/bigquery/_helpers.py

@@ -254,14 +254,15 @@ def bytes_to_py(self, value, field):
         if _not_null(value, field):
             return base64.standard_b64decode(_to_bytes(value))
 
-    def timestamp_to_py(self, value, field):
+    def timestamp_to_py(self, value, field) -> datetime.datetime | str | None:
         """Coerce 'value' to a datetime, if set or not nullable. If timestamp
         is of picosecond precision, preserve the string format."""
         if field.timestamp_precision == enums.TimestampPrecision.PICOSECOND:
             return value
         if _not_null(value, field):
             # value will be a integer in seconds, to microsecond precision, in UTC.
             return _datetime_from_microseconds(int(value))
+        return None
 
     def datetime_to_py(self, value, field):
         """Coerce 'value' to a datetime, if set or not nullable.

google/cloud/bigquery/_job_helpers.py

@@ -287,16 +287,15 @@ def _to_query_request(
     # Default to standard SQL.
     request_body.setdefault("useLegacySql", False)
 
-    # Since jobs.query can return results, ensure we use the lossless timestamp
-    # format. See: https://github.com/googleapis/python-bigquery/issues/395
     request_body.setdefault("formatOptions", {})
-    request_body["formatOptions"]["useInt64Timestamp"] = True  # type: ignore
 
+    # Cannot specify both use_int64_timestamp and timestamp_output_format.
     if timestamp_precision == enums.TimestampPrecision.PICOSECOND:
-        # Cannot specify both use_int64_timestamp and timestamp_output_format.
-        del request_body["formatOptions"]["useInt64Timestamp"]
-
         request_body["formatOptions"]["timestampOutputFormat"] = "ISO8601_STRING"
+    else:
+        # Since jobs.query can return results, ensure we use the lossless
+        # timestamp format. See: https://github.com/googleapis/python-bigquery/issues/395
+        request_body["formatOptions"]["useInt64Timestamp"] = True
 
     if timeout is not None:
         # Subtract a buffer for context switching, network latency, etc.

google/cloud/bigquery/client.py

@@ -3552,7 +3552,7 @@ def query(
 
         if (
             timestamp_precision == enums.TimestampPrecision.PICOSECOND
-            and api_method == enums.QueryApiMethod.INSERT
+            and api_method != enums.QueryApiMethod.QUERY
         ):
             raise ValueError(
                 "Picosecond Timestamp is only supported when `api_method "
@@ -4167,13 +4167,11 @@ def list_rows(
         if start_index is not None:
             params["startIndex"] = start_index
 
-        params["formatOptions.useInt64Timestamp"] = True
-
+        # Cannot specify both use_int64_timestamp and timestamp_output_format.
         if timestamp_precision == enums.TimestampPrecision.PICOSECOND:
-            # Cannot specify both use_int64_timestamp and timestamp_output_format.
-            del params["formatOptions.useInt64Timestamp"]
-
             params["formatOptions.timestampOutputFormat"] = "ISO8601_STRING"
+        else:
+            params["formatOptions.useInt64Timestamp"] = True
 
         row_iterator = RowIterator(
             client=self,