import os

from django.conf import settings

from django.core import signals

from django.core.exceptions import ImproperlyConfigured

from django.dispatch import dispatcher

from django.utils.functional import curry

__all__ = ('backend', 'connection', 'DatabaseError', 'IntegrityError')

if not settings.DATABASE_ENGINE:

    settings.DATABASE_ENGINE = 'dummy'

try:

    # Most of the time, the database backend will be one of the official

    # backends that ships with Django, so look there first.

    _import_path = 'django.db.backends.'

    backend = __import__('%s%s.base' % (_import_path, settings.DATABASE_ENGINE), {}, {}, [''])

    creation = __import__('%s%s.creation' % (_import_path, settings.DATABASE_ENGINE), {}, {}, [''])

except ImportError, e:

    # If the import failed, we might be looking for a database backend

    # distributed external to Django. So we'll try that next.

    try:

        _import_path = ''

        backend = __import__('%s.base' % settings.DATABASE_ENGINE, {}, {}, [''])

        creation = __import__('%s.creation' % settings.DATABASE_ENGINE, {}, {}, [''])

    except ImportError, e_user:

        # The database backend wasn't found. Display a helpful error message

        # listing all possible (built-in) database backends.

        backend_dir = os.path.join(__path__[0], 'backends')

        available_backends = [f for f in os.listdir(backend_dir) if not f.startswith('_') and not f.startswith('.') and not f.endswith('.py') and not f.endswith('.pyc')]

        available_backends.sort()

        if settings.DATABASE_ENGINE not in available_backends:

            raise ImproperlyConfigured, "%r isn't an available database backend. Available options are: %s" % \

                (settings.DATABASE_ENGINE, ", ".join(map(repr, available_backends)))

        else:

            raise # If there's some other error, this must be an error in Django itself.

def _import_database_module(import_path='', module_name=''):

    """Lazily import a database module when requested."""

    return __import__('%s%s.%s' % (import_path, settings.DATABASE_ENGINE, module_name), {}, {}, [''])

# We don't want to import the introspect module unless someone asks for it, so

# lazily load it on demmand.

get_introspection_module = curry(_import_database_module, _import_path, 'introspection')

def get_creation_module():

    return creation

# We want runshell() to work the same way, but we have to treat it a

# little differently (since it just runs instead of returning a module like

# the above) and wrap the lazily-loaded runshell() method.

runshell = lambda: _import_database_module(_import_path, "client").runshell()

# Convenient aliases for backend bits.

connection = backend.DatabaseWrapper(**settings.DATABASE_OPTIONS)

DatabaseError = backend.DatabaseError

IntegrityError = backend.IntegrityError

# Register an event that closes the database connection

# when a Django request is finished.

dispatcher.connect(connection.close, signal=signals.request_finished)

# Register an event that resets connection.queries

# when a Django request is started.

def reset_queries():

    connection.queries = []

dispatcher.connect(reset_queries, signal=signals.request_started)

# Register an event that rolls back the connection

# when a Django request has an exception.

def _rollback_on_exception():

    from django.db import transaction

    transaction.rollback_unless_managed()

dispatcher.connect(_rollback_on_exception, signal=signals.got_request_exception)