diff --git a/README.md b/README.md index aba2401..29b4eb2 100644 --- a/README.md +++ b/README.md @@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/). For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter). -_1833 TILs and counting..._ +_1834 TILs and counting..._ See some of the other learning resources I work on: @@ -1076,6 +1076,7 @@ If you've learned something here, support my efforts writing daily TILs by - [Break Debugger On First Line Of Program](python/break-debugger-on-first-line-of-program.md) - [Check If Package Is Installed With Pip](python/check-if-package-is-installed-with-pip.md) - [Check Precondition Before Click Arg Parsing](python/check-precondition-before-click-arg-parsing.md) +- [Commit Writes From Executed SQLite Statements](python/commit-writes-from-executed-sqlite-statements.md) - [Control Passing Of Time In Tests](python/control-passing-of-time-in-tests.md) - [Create A Dummy DataFrame In Pandas](python/create-a-dummy-dataframe-in-pandas.md) - [Create A Range Of Descending Values](python/create-a-range-of-descending-values.md) diff --git a/python/commit-writes-from-executed-sqlite-statements.md b/python/commit-writes-from-executed-sqlite-statements.md new file mode 100644 index 0000000..0511c09 --- /dev/null +++ b/python/commit-writes-from-executed-sqlite-statements.md @@ -0,0 +1,72 @@ +# Commit Writes From Executed SQLite Statements + +Let's look at a method that uses a +[`sqlite3`](https://docs.python.org/3/library/sqlite3.html) connection to +execute a couple statements against a SQLite database. + +```python +def write_session_with_project(self, session, *, active=False) -> None: + # Delete the current active session if there is one + self.conn.execute(""" + delete from sessions where active = 1; + """) + + # Upsert (find or create) the project based on `session.project_name` + cursor = self.conn.execute( + """ + insert into projects (name) values (:project_name) + on conflict (name) do update set name = excluded.name + returning id; + """, + {"project_name": session.project_name}, + ) + project_id = cursor.fetchone()[0] + + # ... + + self.conn.commit() +``` + +The first `conn.execute` call is going to implicitly start a database +transaction before executing the statement. Subsequent statements are going to +take place within that transaction. To apply all the changes in the transaction +I have to eventually run `conn.commit()`. If there isn't an issue committing all +the changes and nothing else raised before I committed, then those changes will +all be applied atomically. + +I will need to do my own exception handling with a try/catch that handles any +rollback and closes the connection. + +I'd rather not have to manage those extra pieces which is the kind of thing +context managers typically help with. Let's improve upon this with the +[Connection context +manager](https://docs.python.org/3/library/sqlite3.html#how-to-use-the-connection-context-manager): + +```python +def write_session_with_project(self, session, *, active=False) -> None: + with self.conn: + # Delete the current active session if there is one + self.conn.execute(""" + delete from sessions where active = 1; + """) + + # Upsert (find or create) the project based on `session.project_name` + cursor = self.conn.execute( + """ + insert into projects (name) values (:project_name) + on conflict (name) do update set name = excluded.name + returning id; + """, + {"project_name": session.project_name}, + ) + project_id = cursor.fetchone()[0] + + # ... +``` + +I get the same transactional behavior as before for everything in the context +manager body. However, now the `commit` is handled and the `rollback` and +`close` is handled if there is an exception. + +Note: the connection context manager will still re-propagate an exception that +occurred, so I may need to handle that with a try/catch somewhere.