BUG: Register SQLite adapters only once per process (GH#64337)

pandas/io/sql.py

@@ -2544,6 +2544,11 @@ def _quote_identifier(name: object) -> str:
     return '"' + uname.replace('"', '""') + '"'
 
 
+# Guard to avoid overwriting user-registered SQLite adapters/converters
+# on every to_sql() call.  See GH#64337.
+_SQLITE_ADAPTERS_REGISTERED: bool = False
+
+
 class SQLiteTable(SQLTable):
     """
     Patch the SQLTable for fallback support.
@@ -2556,10 +2561,16 @@ def __init__(self, *args, **kwargs) -> None:
         self._register_date_adapters()
 
     def _register_date_adapters(self) -> None:
-        # GH 8341
-        # register an adapter callable for datetime.time object
+        # GH 8341 — register adapter callable for datetime.time object
+        # GH 61092 — register adapters for date/datetime (Python 3.12+)
+        # GH 64337 — only register once per process to avoid overwriting
+        # user-registered adapters on subsequent to_sql() calls.
         import sqlite3
 
+        global _SQLITE_ADAPTERS_REGISTERED
+        if _SQLITE_ADAPTERS_REGISTERED:
+            return
+
         # this will transform time(12,34,56,789) into '12:34:56.000789'
         # (this is what sqlalchemy does)
         def _adapt_time(t) -> str:
@@ -2584,6 +2595,8 @@ def _adapt_time(t) -> str:
         sqlite3.register_converter("date", convert_date)
         sqlite3.register_converter("timestamp", convert_timestamp)
 
+        _SQLITE_ADAPTERS_REGISTERED = True
+
     def sql_schema(self) -> str:
         return str(";\n".join(self.table))
 

pandas/tests/io/test_sql.py

@@ -4417,3 +4417,29 @@ def test_xsqlite_if_exists(sqlite_buildin):
         (5, "E"),
     ]
     drop_table(table_name, sqlite_buildin)
+
+
+def test_to_sql_preserves_user_sqlite_adapters(sqlite_buildin):
+    """
+    GH#64337 — DataFrame.to_sql() should not overwrite user-registered
+    SQLite adapters on subsequent calls.
+    """
+    import sqlite3
+
+    # First to_sql call — pandas registers its adapters
+    df = DataFrame({"t": [time(12, 30)]})
+    df.to_sql("test_adapter_1", sqlite_buildin, if_exists="replace", index=False)
+
+    # User registers a custom adapter for time
+    def custom_time_adapter(t):
+        return f"CUSTOM:{t.hour:02d}:{t.minute:02d}:{t.second:02d}"
+
+    sqlite3.register_adapter(time, custom_time_adapter)
+
+    # Second to_sql call — must NOT overwrite the user's adapter
+    df2 = DataFrame({"t": [time(14, 0)]})
+    df2.to_sql("test_adapter_2", sqlite_buildin, if_exists="replace", index=False)
+
+    # Verify user's adapter is still registered
+    registered = sqlite3.adapters.get((time, sqlite3.PrepareProtocol))
+    assert registered is custom_time_adapter