msqlite

SQLite access that is safe across threads and processes. msqlite takes the lock for you, retries with jittered backoff when another connection holds it, and gives up with a clear error at a deadline instead of hanging or raising database is locked at random. It is pure Python with no dependencies: one module on top of the standard library's sqlite3.

• Pure Python, zero dependencies. No compiled extension, so one wheel installs on every platform and there is nothing to build. Ships py.typed and passes mypy strict.
• Database: multi-table access with write() and read() transactions, WAL by default so readers never wait for a writer, and one reused connection per thread.
• MSQLite: the original single-table helper with an auto-created schema. Unchanged API.
• Python 3.12 and later.

pip install msqlite

Quick start

from pathlib import Path
from msqlite import Database

db = Database(Path("app.sqlite"))  # WAL, 30 s lock deadline, one connection per thread

db.migrate([
    (1, "CREATE TABLE journal(id INTEGER PRIMARY KEY, ts REAL, entry TEXT);"),
    (2, "CREATE INDEX journal_ts ON journal(ts);"),
])

with db.write() as tx:                       # BEGIN IMMEDIATE, retried until the deadline
    tx.execute("INSERT INTO journal(ts, entry) VALUES (?, ?)", (1.0, "started"))
    tx.executemany("INSERT INTO journal(ts, entry) VALUES (?, ?)", [(2.0, "a"), (3.0, "b")])
    print(tx.lastrowid, tx.rowcount)

with db.read() as tx:                        # BEGIN DEFERRED; never blocks on a writer under WAL
    rows = tx.execute("SELECT entry FROM journal ORDER BY ts").fetchall()

db.close()                                   # or use `with Database(...) as db:`

A transaction commits when the with block exits normally and rolls back if an exception propagates. tx exposes execute, executemany, executescript, fetchone, fetchall, lastrowid, rowcount, row_factory and the raw connection.

The single-table helper

MSQLite opens a fresh connection per with block, holds an EXCLUSIVE transaction for the whole block, and creates its table on first use from a schema dict. It is the right tool for a small status or results table that several processes write to.

import time
from pathlib import Path
from msqlite import MSQLite

table_name = "example"
schema = {"id PRIMARY KEY": int, "name": str, "color": str, "year": int}
db_path = Path("temp", "example.sqlite")
db_path.parent.mkdir(exist_ok=True)

with MSQLite(db_path, table_name, schema) as db:
    now = time.monotonic_ns()  # some index value
    db.execute(f"INSERT INTO {table_name} VALUES (?, ?, ?, ?), (?, ?, ?, ?)", (now, "plate", "red", 2020, now + 1, "chair", "green", 2019))
    for row in db.execute(f"SELECT * FROM {table_name}"):
        print(row)

with MSQLite(db_path, table_name) as db:  # reading an existing table needs no schema
    for row in db.execute(f"SELECT * FROM {table_name}"):
        print(row)

Column types map as int→INTEGER, float→REAL, str→TEXT, bytes→BLOB, bool→INTEGER, json→JSON and None→NULL. Constraints go after the column name in the key, as in "id PRIMARY KEY". indexes=["name"] creates one index per listed column.

Concurrency model

SQLite allows one writer at a time per database file. Everything msqlite does is about acquiring that write lock predictably:

1. Take the lock at BEGIN. A write transaction starts with BEGIN IMMEDIATE (or BEGIN EXCLUSIVE), so the lock is acquired before any statement runs and lock errors happen in one predictable place.
2. Let SQLite wait first. Each connection has a busy timeout (busy_timeout_s, default 5 s). SQLite's own busy handler absorbs short waits inside the engine without a Python round trip.
3. Then retry in Python. A lock error (SQLITE_BUSY or SQLITE_LOCKED, detected by result code, not message text) is retried with jittered exponential backoff: the first sleep is uniform in [0, 2 * retry_scale) and the range doubles per retry up to retry_cap_s.
4. Stop at a deadline. timeout_s bounds the total time spent waiting for locks within one transaction. When it is exceeded, MSQLiteTimeoutError(elapsed_s, attempts) is raised. retry_limit bounds the number of retries instead, raising MSQLiteMaxRetriesError, a subclass of MSQLiteTimeoutError. The busy timeout is capped to the time remaining before the deadline, so a single attempt cannot overshoot it.
5. Retry after BEGIN too. A statement inside the transaction, or the COMMIT itself, can still hit a lock error (in rollback-journal mode a commit waits for readers). These are retried in place: only the failed statement is re-executed and the transaction stays open, so earlier statements are kept. executemany and executescript are not retried, since a partial run could not be re-applied safely; inside a write transaction they already hold the write lock, so they do not hit lock errors in practice.

Database keeps one connection per thread and reuses it across transactions; a thread's connection is closed when the thread ends. Connections are never shared between threads and never carried across a fork (a forked child opens its own). MSQLite opens and closes a connection per with block, which is what makes it process-safe with zero setup.

The three transaction modes

Why IMMEDIATE for writes. A DEFERRED transaction that reads and then writes must upgrade its shared lock to a write lock mid-transaction. If another writer is waiting, both are stuck: the writer waits for readers to leave, and the reader waits for the writer. SQLite reports SQLITE_BUSY without invoking the busy handler, and under WAL the upgrade fails outright with SQLITE_BUSY_SNAPSHOT once another writer has committed, because the transaction's snapshot is stale. Retrying cannot help, so msqlite raises that error immediately. IMMEDIATE takes the write lock up front and the case never arises. Rule: anything that writes goes in db.write().

Under WAL (the Database default) readers work from a snapshot and never wait for a writer, and a writer never waits for readers. PRAGMA journal_mode=WAL is persisted in the database file once set, so files created by Database stay in WAL mode for every other tool that opens them. WAL creates -wal and -shm files next to the database.

When a rollback journal is still right. WAL needs shared memory between the processes that open the file, so it does not work over network filesystems (NFS, SMB shares). If the database lives on one of those, or if there is a single writer and readers can tolerate waiting, use a rollback journal: Database(path, wal=False) (which leaves an existing file's journal mode untouched). MSQLite leaves it alone unless asked (wal=True), so existing databases see no change.

Configuration

Database(
    path,
    wal=True,               # PRAGMA journal_mode=WAL on every connection
    timeout_s=30.0,         # total lock wait per transaction; None waits forever
    retry_limit=None,       # retries per transaction; None is unbounded
    retry_scale=0.01,       # base backoff unit in seconds
    retry_cap_s=1.0,        # upper bound of one backoff sleep
    busy_timeout_s=5.0,     # SQLite's own busy timeout per attempt
    persistent=True,        # one connection per thread, reused; False opens one per transaction
    # The rest default to off; these are examples.
    pragmas=("synchronous=FULL",),                                  # applied to every new connection
    on_open=lambda conn: conn.execute("PRAGMA foreign_keys=ON"),    # after the pragmas
    on_retry=lambda attempt, elapsed_s, error: log.warning("retry %d after %.2fs: %s", attempt, elapsed_s, error),
    row_factory=sqlite3.Row,                                        # applied to every cursor
)

All arguments after the path are keyword-only. A pragma may be written with or without the PRAGMA prefix; one containing a ; is rejected.

MSQLite(path, table, schema, indexes, *, retry_scale, retry_limit, timeout_s, retry_cap_s, busy_timeout_s, wal, mode, on_retry) takes the same knobs. Its defaults keep the historical behaviour: rollback journal, EXCLUSIVE, no deadline.

Migrations

db.migrate(steps) applies (version, sql_script) pairs keyed on PRAGMA user_version and returns the resulting version. Versions must be positive and strictly increasing (ValueError otherwise). Steps at or below the current version are skipped; the rest run in one write transaction, so a failing step rolls everything back and leaves user_version untouched. Projects with their own migrations table can ignore it.

Observability

db.stats (and MSQLite.stats) is a Stats dataclass with executions, retries, max_execution_s and max_wait_s. on_retry(attempt, elapsed_s, error) is called before each backoff sleep. Log records go to the msqlite logger.

Errors

MSQLiteTimeoutError subclasses sqlite3.OperationalError, so existing except sqlite3.OperationalError handlers keep working. The transaction is rolled back and, for MSQLite, the connection closed before the error reaches the caller. Any other sqlite3 error passes through untouched.

Benchmark

Write latency for one single-row insert per transaction while N processes contend for the same file. Each process does 20 writes; the table shows the latency distribution of the with block (lock acquisition, the insert and the commit) and the aggregate throughput.

Measured on Windows-11-10.0.26300-SP0, Python 3.14.9, SQLite 3.50.4 (a desktop with an NVMe SSD; numbers on CI runners or laptops will differ):

Per-write latency is dominated by the commit's fsync and, under contention, by waiting for the other writers: with 100 processes each write queues behind up to 99 others. The rollback-journal helper pays for opening a connection per access and for EXCLUSIVE locking. Reproduce with python scripts/benchmark.py.

When not to use this

• Many concurrent writers with high throughput needs. SQLite serializes writers; past a few hundred small writes per second across processes, a client-server database is the better fit.
• Network filesystems. SQLite locking is unreliable on NFS and SMB, WAL does not work there at all, and retrying cannot fix a lock that the filesystem reports incorrectly.
• Long read-modify-write sequences in one transaction. The write lock is held for the whole with block. Keep transactions short or every other writer waits.
• Code that needs a connection pool or async access. msqlite is synchronous and hands out one connection per thread.

Development

The project is managed with uv: pyproject.toml holds the dev dependency group and uv.lock pins it.

uv sync --no-build             # .venv with the package (editable) and the dev group, wheels only
uv run pytest                  # includes a 200-process contention test; -m "not slow" skips the slowest
uv run ruff format src test_msqlite scripts && uv run ruff check src test_msqlite scripts
uv run mypy                    # strict, configured in pyproject.toml
uv build

Tests run on the minimum supported Python, the latest stable release and the upcoming pre-release. Releases are published to PyPI by the GitHub release workflow through trusted publishing. See CHANGELOG.md for the release history.