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sqlalchemy / lib / sqlalchemy / dialects / sqlite / pysqlite.py

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# sqlite/pysqlite.py
# Copyright (C) 2005-2011 the SQLAlchemy authors and contributors <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Support for the SQLite database via pysqlite.

Note that pysqlite is the same driver as the ``sqlite3``
module included with the Python distribution.

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 :func:`~sqlalchemy.create_engine()` to control 
this explicitly::

    from sqlite3 import dbapi2 as sqlite
    e = create_engine('sqlite+pysqlite:///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://')

Compatibility with sqlite3 "native" date and datetime types
-----------------------------------------------------------

The pysqlite driver includes the sqlite3.PARSE_DECLTYPES and 
sqlite3.PARSE_COLNAMES options, which have the effect of any column
or expression explicitly cast as "date" or "timestamp" will be converted
to a Python date or datetime object.  The date and datetime types provided 
with the pysqlite dialect are not currently compatible with these options, 
since they render the ISO date/datetime including microseconds, which 
pysqlite's driver does not.   Additionally, SQLAlchemy does not at
this time automatically render the "cast" syntax required for the 
freestanding functions "current_timestamp" and "current_date" to return
datetime/date types natively.   Unfortunately, pysqlite 
does not provide the standard DBAPI types in ``cursor.description``,
leaving SQLAlchemy with no way to detect these types on the fly 
without expensive per-row type checks.

Keeping in mind that pysqlite's parsing option is not recommended,
nor should be necessary, for use with SQLAlchemy, usage of PARSE_DECLTYPES 
can be forced if one configures "native_datetime=True" on create_engine()::

    engine = create_engine('sqlite://', 
                    connect_args={'detect_types': sqlite3.PARSE_DECLTYPES|sqlite3.PARSE_COLNAMES},
                    native_datetime=True
                    )

With this flag enabled, the DATE and TIMESTAMP types (but note - not the DATETIME
or TIME types...confused yet ?) will not perform any bind parameter or result
processing. Execution of "func.current_date()" will return a string.
"func.current_timestamp()" is registered as returning a DATETIME type in
SQLAlchemy, so this function still receives SQLAlchemy-level result processing.

Threading/Pooling Behavior
---------------------------

Pysqlite's default behavior is to prohibit the usage of a single connection
in more than one thread.   This is controlled by the ``check_same_thread``
Pysqlite flag.   This default is intended to work with older versions
of SQLite that did not support multithreaded operation under 
various circumstances.  In particular, older SQLite versions
did not allow a ``:memory:`` database to be used in multiple threads
under any circumstances.

SQLAlchemy sets up pooling to work with Pysqlite's default behavior:

* When a ``:memory:`` SQLite database is specified, the dialect by default will use
  :class:`.SingletonThreadPool`. This pool maintains a single connection per
  thread, so that all access to the engine within the current thread use the
  same ``:memory:`` database - other threads would access a different 
  ``:memory:`` database.
* When a file-based database is specified, the dialect will use :class:`.NullPool` 
  as the source of connections. This pool closes and discards connections
  which are returned to the pool immediately. SQLite file-based connections
  have extremely low overhead, so pooling is not necessary. The scheme also
  prevents a connection from being used again in a different thread and works
  best with SQLite's coarse-grained file locking.

  .. note:: 
  
     The default selection of :class:`.NullPool` for SQLite file-based databases 
     is new in SQLAlchemy 0.7. Previous versions
     select :class:`.SingletonThreadPool` by
     default for all SQLite databases.

Modern versions of SQLite no longer have the threading restrictions, and assuming
the sqlite3/pysqlite library was built with SQLite's default threading mode
of "Serialized", even ``:memory:`` databases can be shared among threads.

Using a Memory Database in Multiple Threads
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To use a ``:memory:`` database in a multithreaded scenario, the same connection
object must be shared among threads, since the database exists
only within the scope of that connection.   The :class:`.StaticPool` implementation
will maintain a single connection globally, and the ``check_same_thread`` flag
can be passed to Pysqlite as ``False``::

    from sqlalchemy.pool import StaticPool
    engine = create_engine('sqlite://',
                        connect_args={'check_same_thread':False},
                        poolclass=StaticPool)

Note that using a ``:memory:`` database in multiple threads requires a recent 
version of SQLite.

Using Temporary Tables with SQLite
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Due to the way SQLite deals with temporary tables, if you wish to use a temporary table
in a file-based SQLite database across multiple checkouts from the connection pool, such
as when using an ORM :class:`.Session` where the temporary table should continue to remain
after :meth:`.commit` or :meth:`.rollback` is called,
a pool which maintains a single connection must be used.   Use :class:`.SingletonThreadPool`
if the scope is only needed within the current thread, or :class:`.StaticPool` is scope is
needed within multiple threads for this case::

    # maintain the same connection per thread
    from sqlalchemy.pool import SingletonThreadPool
    engine = create_engine('sqlite:///mydb.db',
                        poolclass=SingletonThreadPool)


    # maintain the same connection across all threads
    from sqlalchemy.pool import StaticPool
    engine = create_engine('sqlite:///mydb.db',
                        poolclass=StaticPool)

Note that :class:`.SingletonThreadPool` should be configured for the number of threads
that are to be used; beyond that number, connections will be closed out in a non deterministic
way.

Unicode
-------

The pysqlite driver only returns Python ``unicode`` objects in result sets, never
plain strings, and accommodates ``unicode`` objects within bound parameter
values in all cases.   Regardless of the SQLAlchemy string type in use, 
string-based result values will by Python ``unicode`` in Python 2.  
The :class:`.Unicode` type should still be used to indicate those columns that
require unicode, however, so that non-``unicode`` values passed inadvertently
will emit a warning.  Pysqlite will emit an error if a non-``unicode`` string
is passed containing non-ASCII characters.

.. _pysqlite_serializable:

Serializable Transaction Isolation
----------------------------------

The pysqlite DBAPI driver has a long-standing bug in which transactional
state is not begun until the first DML statement, that is INSERT, UPDATE
or DELETE, is emitted.  A SELECT statement will not cause transactional
state to begin.   While this mode of usage is fine for typical situations
and has the advantage that the SQLite database file is not prematurely 
locked, it breaks serializable transaction isolation, which requires
that the database file be locked upon any SQL being emitted.

To work around this issue, the ``BEGIN`` keyword can be emitted
at the start of each transaction.   The following recipe establishes
a :meth:`.ConnectionEvents.begin` handler to achieve this::

    from sqlalchemy import create_engine, event

    engine = create_engine("sqlite:///myfile.db", isolation_level='SERIALIZABLE')

    @event.listens_for(engine, "begin")
    def do_begin(conn):
        conn.execute("BEGIN")

"""

from sqlalchemy.dialects.sqlite.base import SQLiteDialect, DATETIME, DATE
from sqlalchemy import exc, pool
from sqlalchemy import types as sqltypes
from sqlalchemy import util

import os

class _SQLite_pysqliteTimeStamp(DATETIME):
    def bind_processor(self, dialect):
        if dialect.native_datetime:
            return None
        else:
            return DATETIME.bind_processor(self, dialect)

    def result_processor(self, dialect, coltype):
        if dialect.native_datetime:
            return None
        else:
            return DATETIME.result_processor(self, dialect, coltype)

class _SQLite_pysqliteDate(DATE):
    def bind_processor(self, dialect):
        if dialect.native_datetime:
            return None
        else:
            return DATE.bind_processor(self, dialect)

    def result_processor(self, dialect, coltype):
        if dialect.native_datetime:
            return None
        else:
            return DATE.result_processor(self, dialect, coltype)

class SQLiteDialect_pysqlite(SQLiteDialect):
    default_paramstyle = 'qmark'

    colspecs = util.update_copy(
        SQLiteDialect.colspecs,
        {
            sqltypes.Date:_SQLite_pysqliteDate,
            sqltypes.TIMESTAMP:_SQLite_pysqliteTimeStamp,
        }
    )

    # Py3K
    #description_encoding = None

    driver = 'pysqlite'

    def __init__(self, **kwargs):
        SQLiteDialect.__init__(self, **kwargs)

        if self.dbapi is not None:
            sqlite_ver = self.dbapi.version_info
            if sqlite_ver < (2, 1, 3):
                util.warn(
                    ("The installed version of pysqlite2 (%s) is out-dated "
                     "and will cause errors in some cases.  Version 2.1.3 "
                     "or greater is recommended.") %
                    '.'.join([str(subver) for subver in sqlite_ver]))

    @classmethod
    def dbapi(cls):
        try:
            from pysqlite2 import dbapi2 as sqlite
        except ImportError, e:
            try:
                from sqlite3 import dbapi2 as sqlite #try the 2.5+ stdlib name.
            except ImportError:
                raise e
        return sqlite

    @classmethod
    def get_pool_class(cls, url):
        if url.database and url.database != ':memory:':
            return pool.NullPool
        else:
            return pool.SingletonThreadPool

    def _get_server_version_info(self, connection):
        return self.dbapi.sqlite_version_info

    def create_connect_args(self, url):
        if url.username or url.password or url.host or url.port:
            raise exc.ArgumentError(
                "Invalid SQLite URL: %s\n"
                "Valid SQLite URL forms are:\n"
                " sqlite:///:memory: (or, sqlite://)\n"
                " sqlite:///relative/path/to/file.db\n"
                " sqlite:////absolute/path/to/file.db" % (url,))
        filename = url.database or ':memory:'
        if filename != ':memory:':
            filename = os.path.abspath(filename)

        opts = url.query.copy()
        util.coerce_kw_type(opts, 'timeout', float)
        util.coerce_kw_type(opts, 'isolation_level', str)
        util.coerce_kw_type(opts, 'detect_types', int)
        util.coerce_kw_type(opts, 'check_same_thread', bool)
        util.coerce_kw_type(opts, 'cached_statements', int)

        return ([filename], opts)

    def is_disconnect(self, e, connection, cursor):
        return isinstance(e, self.dbapi.ProgrammingError) and \
                "Cannot operate on a closed database." in str(e)

dialect = SQLiteDialect_pysqlite