REGR: fix roundtripping datetimes with sqlite type detection (#55690)

Co-authored-by: Matthew Roeschke <[email protected]>

Author: jorisvandenbossche

doc/source/whatsnew/v2.1.2.rst

@@ -28,6 +28,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrameGroupBy.agg` and :meth:`SeriesGroupBy.agg` where if the option ``compute.use_numba`` was set to True, groupby methods not supported by the numba engine would raise a ``TypeError`` (:issue:`55520`)
 - Fixed performance regression with wide DataFrames, typically involving methods where all columns were accessed individually (:issue:`55256`, :issue:`55245`)
 - Fixed regression in :func:`merge_asof` raising ``TypeError`` for ``by`` with datetime and timedelta dtypes (:issue:`55453`)
+- Fixed regression in :meth:`DataFrame.to_sql` not roundtripping datetime columns correctly for sqlite when using ``detect_types`` (:issue:`55554`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_212.bug_fixes:

pandas/io/sql.py

@@ -2094,19 +2094,17 @@ def _adapt_time(t) -> str:
         # Python 3.12+ doesn't auto-register adapters for us anymore
 
         adapt_date_iso = lambda val: val.isoformat()
-        adapt_datetime_iso = lambda val: val.isoformat()
+        adapt_datetime_iso = lambda val: val.isoformat(" ")
 
         sqlite3.register_adapter(time, _adapt_time)
 
         sqlite3.register_adapter(date, adapt_date_iso)
         sqlite3.register_adapter(datetime, adapt_datetime_iso)
 
         convert_date = lambda val: date.fromisoformat(val.decode())
-        convert_datetime = lambda val: datetime.fromisoformat(val.decode())
-        convert_timestamp = lambda val: datetime.fromtimestamp(int(val))
+        convert_timestamp = lambda val: datetime.fromisoformat(val.decode())
 
         sqlite3.register_converter("date", convert_date)
-        sqlite3.register_converter("datetime", convert_datetime)
         sqlite3.register_converter("timestamp", convert_timestamp)
 
     def sql_schema(self) -> str:

pandas/tests/io/test_sql.py

@@ -3368,6 +3368,24 @@ def test_roundtripping_datetimes(sqlite_engine):
     assert result == "2020-12-31 12:00:00.000000"
 
 
+@pytest.fixture
+def sqlite_builtin_detect_types():
+    with contextlib.closing(
+        sqlite3.connect(":memory:", detect_types=sqlite3.PARSE_DECLTYPES)
+    ) as closing_conn:
+        with closing_conn as conn:
+            yield conn
+
+
+def test_roundtripping_datetimes_detect_types(sqlite_builtin_detect_types):
+    # https://github.com/pandas-dev/pandas/issues/55554
+    conn = sqlite_builtin_detect_types
+    df = DataFrame({"t": [datetime(2020, 12, 31, 12)]}, dtype="datetime64[ns]")
+    df.to_sql("test", conn, if_exists="replace", index=False)
+    result = pd.read_sql("select * from test", conn).iloc[0, 0]
+    assert result == Timestamp("2020-12-31 12:00:00.000000")
+
+
 @pytest.mark.db
 def test_psycopg2_schema_support(postgresql_psycopg2_engine):
     conn = postgresql_psycopg2_engine