import os
from threading import local

from django.conf import settings
from django.core.exceptions import ImproperlyConfigured
from django.utils.importlib import import_module


DEFAULT_DB_ALIAS = 'default'

# Define some exceptions that mirror the PEP249 interface.
# We will rethrow any backend-specific errors using these
# common wrappers
class DatabaseError(Exception):
    pass

class IntegrityError(DatabaseError):
    pass


def load_backend(backend_name):
    # Look for a fully qualified database backend name
    try:
        return import_module('.base', backend_name)
    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(os.path.dirname(__file__), 'backends')
        try:
            available_backends = [f for f in os.listdir(backend_dir)
                    if os.path.isdir(os.path.join(backend_dir, f))
                    and not f.startswith('.')]
        except EnvironmentError:
            available_backends = []
        if backend_name.startswith('django.db.backends.'):
            backend_name = backend_name[19:] # See #15621.
        if backend_name not in available_backends:
            error_msg = ("%r isn't an available database backend. \n" +
                "Try using django.db.backends.XXX, where XXX is one of:\n    %s\n" +
                "Error was: %s") % \
                (backend_name, ", ".join(map(repr, sorted(available_backends))), e_user)
            raise ImproperlyConfigured(error_msg)
        else:
            raise # If there's some other error, this must be an error in Django itself.


class ConnectionDoesNotExist(Exception):
    pass


class ConnectionHandler(object):
    def __init__(self, databases):
        self.databases = databases
        self._connections = local()

    def ensure_defaults(self, alias):
        """
        Puts the defaults into the settings dictionary for a given connection
        where no settings is provided.
        """
        try:
            conn = self.databases[alias]
        except KeyError:
            raise ConnectionDoesNotExist("The connection %s doesn't exist" % alias)

        conn.setdefault('ENGINE', 'django.db.backends.dummy')
        if conn['ENGINE'] == 'django.db.backends.' or not conn['ENGINE']:
            conn['ENGINE'] = 'django.db.backends.dummy'
        conn.setdefault('OPTIONS', {})
        conn.setdefault('TIME_ZONE', 'UTC' if settings.USE_TZ else settings.TIME_ZONE)
        for setting in ['NAME', 'USER', 'PASSWORD', 'HOST', 'PORT']:
            conn.setdefault(setting, '')
        for setting in ['TEST_CHARSET', 'TEST_COLLATION', 'TEST_NAME', 'TEST_MIRROR']:
            conn.setdefault(setting, None)

    def __getitem__(self, alias):
        if hasattr(self._connections, alias):
            return getattr(self._connections, alias)

        self.ensure_defaults(alias)
        db = self.databases[alias]
        backend = load_backend(db['ENGINE'])
        conn = backend.DatabaseWrapper(db, alias)
        setattr(self._connections, alias, conn)
        return conn

    def __setitem__(self, key, value):
        setattr(self._connections, key, value)

    def __iter__(self):
        return iter(self.databases)

    def all(self):
        return [self[alias] for alias in self]


class ConnectionRouter(object):
    def __init__(self, routers):
        self.routers = []
        for r in routers:
            if isinstance(r, basestring):
                try:
                    module_name, klass_name = r.rsplit('.', 1)
                    module = import_module(module_name)
                except ImportError, e:
                    raise ImproperlyConfigured('Error importing database router %s: "%s"' % (klass_name, e))
                try:
                    router_class = getattr(module, klass_name)
                except AttributeError:
                    raise ImproperlyConfigured('Module "%s" does not define a database router name "%s"' % (module, klass_name))
                else:
                    router = router_class()
            else:
                router = r
            self.routers.append(router)

    def _router_func(action):
        def _route_db(self, model, **hints):
            chosen_db = None
            for router in self.routers:
                try:
                    method = getattr(router, action)
                except AttributeError:
                    # If the router doesn't have a method, skip to the next one.
                    pass
                else:
                    chosen_db = method(model, **hints)
                    if chosen_db:
                        return chosen_db
            try:
                return hints['instance']._state.db or DEFAULT_DB_ALIAS
            except KeyError:
                return DEFAULT_DB_ALIAS
        return _route_db

    db_for_read = _router_func('db_for_read')
    db_for_write = _router_func('db_for_write')

    def allow_relation(self, obj1, obj2, **hints):
        for router in self.routers:
            try:
                method = router.allow_relation
            except AttributeError:
                # If the router doesn't have a method, skip to the next one.
                pass
            else:
                allow = method(obj1, obj2, **hints)
                if allow is not None:
                    return allow
        return obj1._state.db == obj2._state.db

    def allow_syncdb(self, db, model):
        for router in self.routers:
            try:
                method = router.allow_syncdb
            except AttributeError:
                # If the router doesn't have a method, skip to the next one.
                pass
            else:
                allow = method(db, model)
                if allow is not None:
                    return allow
        return True
