SQLite — SQLAlchemy 0.5.0 Documentation

# SQLAlchemy 0.5.0 Documentation

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Version: 0.5.0 Last Updated: 01/06/2009 13:41:02

# SQLite

Support for the SQLite database.

## Driver

When using Python 2.5 and above, the built in `sqlite3` driver is already installed and no additional installation is needed. Otherwise, the `pysqlite2` driver needs to be present. This is the same driver as `sqlite3`, just with a different name.

The `pysqlite2` driver will be loaded first, and if not found, `sqlite3` is loaded. This allows an explicitly installed pysqlite driver to take precedence over the built in one. As with all dialects, a specific DBAPI module may be provided to `create_engine()` to control this explicitly:

    from sqlite3 import dbapi2 as sqlite
    e = create_engine('sqlite:///file.db', module=sqlite)

Full documentation on pysqlite is available at: http://www.initd.org/pub/software/pysqlite/doc/usage-guide.html

## Connect Strings

The file specification for the SQLite database is taken as the “database” portion of the URL. Note that the format of a url is:

    driver://user:pass@host/database

This means that the actual filename to be used starts with the characters to the right of the third slash. So connecting to a relative filepath looks like:

    # relative path
    e = create_engine('sqlite:///path/to/database.db')

An absolute path, which is denoted by starting with a slash, means you need four slashes:

    # absolute path
    e = create_engine('sqlite:////path/to/database.db')

To use a Windows path, regular drive specifications and backslashes can be used. Double backslashes are probably needed:

    # absolute path on Windows
    e = create_engine('sqlite:///C:\\path\\to\\database.db')

The sqlite `:memory:` identifier is the default if no filepath is present. Specify `sqlite://` and nothing else:

    # in-memory database
    e = create_engine('sqlite://')

## Threading Behavior

Pysqlite connections do not support being moved between threads, unless the `check_same_thread` Pysqlite flag is set to `False`. In addition, when using an in-memory SQLite database, the full database exists only within the scope of a single connection. It is reported that an in-memory database does not support being shared between threads regardless of the `check_same_thread` flag - which means that a multithreaded application cannot share data from a `:memory:` database across threads unless access to the connection is limited to a single worker thread which communicates through a queueing mechanism to concurrent threads.

To provide a default which accomodates SQLite’s default threading capabilities somewhat reasonably, the SQLite dialect will specify that the `SingletonThreadPool` be used by default. This pool maintains a single SQLite connection per thread that is held open up to a count of five concurrent threads. When more than five threads are used, a cleanup mechanism will dispose of excess unused connections.

Two optional pool implementations that may be appropriate for particular SQLite usage scenarios:

> - the `sqlalchemy.pool.StaticPool` might be appropriate for a multithreaded application using an in-memory database, assuming the threading issues inherent in pysqlite are somehow accomodated for. This pool holds persistently onto a single connection which is never closed, and is returned for all requests.
> - the `sqlalchemy.pool.NullPool` might be appropriate for an application that makes use of a file-based sqlite database. This pool disables any actual “pooling” behavior, and simply opens and closes real connections corresonding to the `connect()` and `close()` methods. SQLite can “connect” to a particular file with very high efficiency, so this option may actually perform better without the extra overhead of `SingletonThreadPool`. NullPool will of course render a `:memory:` connection useless since the database would be lost as soon as the connection is “returned” to the pool.

## Date and Time Types

SQLite does not have built-in DATE, TIME, or DATETIME types, and pysqlite does not provide out of the box functionality for translating values between Python datetime objects and a SQLite-supported format. SQLAlchemy’s own `DateTime` and related types provide date formatting and parsing functionality when SQlite is used. The implementation classes are `SLDateTime`, `SLDate` and `SLTime`. These types represent dates and times as ISO formatted strings, which also nicely support ordering. There’s no reliance on typical “libc” internals for these functions so historical dates are fully supported.

## Unicode

In contrast to SQLAlchemy’s active handling of date and time types for pysqlite, pysqlite’s default behavior regarding Unicode is that all strings are returned as Python unicode objects in all cases. So even if the `Unicode` type is not used, you will still always receive unicode data back from a result set. It is strongly recommended that you do use the `Unicode` type to represent strings, since it will raise a warning if a non-unicode Python string is passed from the user application. Mixing the usage of non-unicode objects with returned unicode objects can quickly create confusion, particularly when using the ORM as internal data is not always represented by an actual database result string.