sqlitedict -- persistent `dict`, backed-up by SQLite and pickle

A lightweight wrapper around Python's sqlite3 database with a simple, Pythonic dict-like interface and support for multi-thread access:

>>> from sqlitedict import SqliteDict
>>> mydict = SqliteDict('./my_db.sqlite', autocommit=True)
>>> mydict['some_key'] = any_picklable_object
>>> print mydict['some_key']  # prints the new value
>>> for key, value in mydict.iteritems():
>>>     print key, value
>>> print len(mydict) # etc... all dict functions work
>>> mydict.close()

Pickle is used internally to (de)serialize the values. Keys are arbitrary strings, values arbitrary pickle-able objects.

If you don't use autocommit (default is no autocommit for performance), then don't forget to call mydict.commit() when done with a transaction:

>>> # using SqliteDict as context manager works too (RECOMMENDED)
>>> with SqliteDict('./my_db.sqlite') as mydict:  # note no autocommit=True
...     mydict['some_key'] = u"first value"
...     mydict['another_key'] = range(10)
...     mydict.commit()
...     mydict['some_key'] = u"new value"
...     # no explicit commit here
>>> with SqliteDict('./my_db.sqlite') as mydict:  # re-open the same DB
...     print mydict['some_key']  # outputs 'first value', not 'new value'

Features

Values can be any picklable objects (uses cPickle with the highest protocol).
Support for multiple tables (=dicts) living in the same database file.
Support for access from multiple threads to the same connection (needed by e.g. Pyro). Vanilla sqlite3 gives you ProgrammingError: SQLite objects created in a thread can only be used in that same thread.

Concurrent requests are still serialized internally, so this "multithreaded support" doesn't give you any performance benefits. It is a work-around for sqlite limitations in Python.

Support for custom serialization or compression:

# use JSON instead of pickle
>>> import json
>>> mydict = SqliteDict('./my_db.sqlite', encode=json.dumps, decode=json.loads)

# apply zlib compression after pickling
>>> import zlib, pickle, sqlite3
>>> def my_encode(obj):
...     return sqlite3.Binary(zlib.compress(pickle.dumps(obj, pickle.HIGHEST_PROTOCOL)))
>>> def my_decode(obj):
...     return pickle.loads(zlib.decompress(bytes(obj)))
>>> mydict = SqliteDict('./my_db.sqlite', encode=my_encode, decode=my_decode)

Installation

The module has no dependencies beyond Python itself. The minimum Python version is 2.5, continuously tested on Python 2.6, 2.7, 3.3 and 3.4 on Travis.

Install or upgrade with:

pip install -U sqlitedict

or from the source tar.gz:

python setup.py install

Documentation

Standard Python document strings are inside the module:

>>> import sqlitedict
>>> help(sqlitedict)

(but it's just dict with a commit, really).

Beware: because of Python semantics, sqlitedict cannot know when a mutable SqliteDict-backed entry was modified in RAM. For example, mydict.setdefault('new_key', []).append(1) will leave mydict['new_key'] equal to empty list, not [1]. You'll need to explicitly assign the mutated object back to SqliteDict to achieve the same effect:

>>> val = mydict.get('new_key', [])
>>> val.append(1)  # sqlite DB not updated here!
>>> mydict['new_key'] = val  # now updated

For developers

Install:

# pip install nose
# pip install coverage

To perform all tests:

# make test-all

To perform all tests with coverage:

# make test-all-with-coverage

Comments, bug reports

sqlitedict resides on github. You can file issues or pull requests there.