Add Register SQLite Adapter To Serialize Datetimes as a Python TIL

README.md

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+- [Register SQLite Adapter To Serialize Datetimes](python/register-sqlite-adapter-to-serialize-datetimes.md)
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python/register-sqlite-adapter-to-serialize-datetimes.md

@@ -0,0 +1,93 @@
+# Register SQLite Adapter To Serialize Datetimes
+
+SQLite doesn't have a datetime or timestamp data type, so that information has
+to be stored as `text` or a unix epoch `int`. That means when using Python's
+`sqlite3` module to perform writes I need to tell it what shape to write a
+datetime value.
+
+I could manually convert `datetime` values everywhere they are involved in
+_write_ operations.
+
+```python
+# Prepare `sessions` insert payload
+session_data = {
+    "active": 1 if active else 0,
+    "project_id": project_id,
+    "start_time": datetime.isoformat(session.start_time),
+    "end_time": None,
+}
+
+if session.end_time:
+    session_data["end_time"] = datetime.isoformat(session.end_time)
+
+# Insert the new active session
+cursor = self.conn.execute(
+    """
+    insert into sessions (active, project_id, start_time, end_time)
+    values (:active, :project_id, :start_time, :end_time)
+    returning id;
+""",
+    session_data,
+)
+```
+
+I've used `datetime.isoformat` above which formats `datetime` objects like so:
+
+```python
+>>> datetime.now().isoformat()
+'2026-08-28T11:52:04.709907'
+```
+
+I'd like to make two improvements.
+
+1. I want these `datetime` values to be formatted instead like `2026-08-28T18:15:27.213Z`.
+2. I want `datetime` values to be serialized automatically in the specific shape
+   without having to manually convert them everywhere.
+
+I can achieve both of these things by [registering an adapter with `sqlite3`](https://docs.python.org/3/library/sqlite3.html#how-to-register-adapter-callables)
+that handles the serialize of `datetime` objects.
+
+First, I define a function that can perform the `datetime` to `str` conversion.
+I decided to put this in `db.py` where I have some other database-specific
+functions.
+
+```python
+from datetime import datetime, timezone
+
+def to_db(dt: datetime) -> str:
+    if dt.tzinfo is None or dt.utcoffset() is None:
+        raise ValueError(f"Unable to store naive datetime: {dt!r}")
+    dt = dt.astimezone(timezone.utc)
+    return f"{dt:%Y-%m-%dT%H:%M:%S}.{dt.microsecond // 1000:03d}Z"
+```
+
+Then I register the adapter before creating the connection that gets used for
+database interactions.
+
+```python
+import sqlite3
+from datetime import datetime
+from pathlib import Path
+from sqlite3 import Connection
+
+def initialize_conn(db_file: Path) -> Connection:
+    # register adapters
+    sqlite3.register_adapter(datetime, to_db)
+
+    conn: Connection = sqlite3.connect(db_file)
+    conn.row_factory = sqlite3.Row
+
+    return conn
+```
+
+Then I can run write operations with `datetime` objects knowing they will be
+correctly serialized.
+
+```python
+with self.conn:
+    query = "update sessions set active = :active, end_time = :end_time where active = 1;"
+    self.conn.execute(
+        query,
+        {"active": 0, "end_time": session.end_time},
+    )
+```