text: import os
  import pkgutil
  from importlib import import_module
  from threading import local
  
  from django.conf import settings
  from django.core.exceptions import ImproperlyConfigured
  from django.utils.functional import cached_property
  from django.utils.module_loading import import_string
  
  DEFAULT_DB_ALIAS = 'default'
  DJANGO_VERSION_PICKLE_KEY = '_django_version'
  
  
  class Error(Exception):
      pass
  
  
  class InterfaceError(Error):
      pass
  
  
  class DatabaseError(Error):
      pass
  
  
  class DataError(DatabaseError):
      pass
  
  
  class OperationalError(DatabaseError):
      pass
  
  
  class IntegrityError(DatabaseError):
      pass
  
  
  class InternalError(DatabaseError):
      pass
  
  
  class ProgrammingError(DatabaseError):
      pass
  
  
  class NotSupportedError(DatabaseError):
      pass
  
  
  class DatabaseErrorWrapper:
      """
      Context manager and decorator that re-throws backend-specific database
      exceptions using Django's common wrappers.
      """
  
      def __init__(self, wrapper):
          """
          wrapper is a database wrapper.
  
          It must have a Database attribute defining PEP-249 exceptions.
          """
          self.wrapper = wrapper
  
      def __enter__(self):
          pass
  
      def __exit__(self, exc_type, exc_value, traceback):
          if exc_type is None:
              return
          for dj_exc_type in (
                  DataError,
                  OperationalError,
                  IntegrityError,
                  InternalError,
                  ProgrammingError,
                  NotSupportedError,
                  DatabaseError,
                  InterfaceError,
                  Error,
          ):
              db_exc_type = getattr(self.wrapper.Database, dj_exc_type.__name__)
              if issubclass(exc_type, db_exc_type):
                  dj_exc_value = dj_exc_type(*exc_value.args)
                  dj_exc_value.__cause__ = exc_value
                  # Only set the 'errors_occurred' flag for errors that may make
                  # the connection unusable.
                  if dj_exc_type not in (DataError, IntegrityError):
                      self.wrapper.errors_occurred = True
                  raise dj_exc_value.with_traceback(traceback)
  
      def __call__(self, func):
          # Note that we are intentionally not using @wraps here for performance
          # reasons. Refs #21109.
          def inner(*args, **kwargs):
              with self:
                  return func(*args, **kwargs)
          return inner
  
  
  def load_backend(backend_name):
      """
      Return a database backend's "base" module given a fully qualified database
      backend name, or raise an error if it doesn't exist.
      """
      # This backend was renamed in Django 1.9.
      if backend_name == 'django.db.backends.postgresql_psycopg2':
          backend_name = 'django.db.backends.postgresql'
  
      try:
          return import_module('%s.base' % backend_name)
      except ImportError as e_user:
          # The database backend wasn't found. Display a helpful error message
          # listing all built-in database backends.
          backend_dir = os.path.join(os.path.dirname(__file__), 'backends')
          builtin_backends = [
              name for _, name, ispkg in pkgutil.iter_modules([backend_dir])
              if ispkg and name not in {'base', 'dummy', 'postgresql_psycopg2'}
          ]
          if backend_name not in ['django.db.backends.%s' % b for b in builtin_backends]:
              backend_reprs = map(repr, sorted(builtin_backends))
              raise ImproperlyConfigured(
                  "%r isn't an available database backend.\n"
                  "Try using 'django.db.backends.XXX', where XXX is one of:\n"
                  "    %s" % (backend_name, ", ".join(backend_reprs))
              ) from e_user
          else:
              # If there's some other error, this must be an error in Django
              raise
  
  
  class ConnectionDoesNotExist(Exception):
      pass
  
  
  class ConnectionHandler:
      def __init__(self, databases=None):
          """
          databases is an optional dictionary of database definitions (structured
          like settings.DATABASES).
          """
          self._databases = databases
          self._connections = local()
  
      @cached_property
      def databases(self):
          if self._databases is None:
              self._databases = settings.DATABASES
          if self._databases == {}:
              self._databases = {
                  DEFAULT_DB_ALIAS: {
                      'ENGINE': 'django.db.backends.dummy',
                  },
              }
          if self._databases[DEFAULT_DB_ALIAS] == {}:
              self._databases[DEFAULT_DB_ALIAS]['ENGINE'] = 'django.db.backends.dummy'
  
          if DEFAULT_DB_ALIAS not in self._databases:
              raise ImproperlyConfigured("You must define a '%s' database" % DEFAULT_DB_ALIAS)
          return self._databases
  
      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('ATOMIC_REQUESTS', False)
          conn.setdefault('AUTOCOMMIT', True)
          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('CONN_MAX_AGE', 0)
          conn.setdefault('OPTIONS', {})
          conn.setdefault('TIME_ZONE', None)
          for setting in ['NAME', 'USER', 'PASSWORD', 'HOST', 'PORT']:
              conn.setdefault(setting, '')
  
      def prepare_test_settings(self, alias):
          """
          Makes sure the test settings are available in the 'TEST' sub-dictionary.
          """
          try:
              conn = self.databases[alias]
          except KeyError:
              raise ConnectionDoesNotExist("The connection %s doesn't exist" % alias)
  
          test_settings = conn.setdefault('TEST', {})
          for key in ['CHARSET', 'COLLATION', 'NAME', 'MIRROR']:
              test_settings.setdefault(key, None)
  
      def __getitem__(self, alias):
          if hasattr(self._connections, alias):
              return getattr(self._connections, alias)
  
          self.ensure_defaults(alias)
          self.prepare_test_settings(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 __delitem__(self, key):
          delattr(self._connections, key)
  
      def __iter__(self):
          return iter(self.databases)
  
      def all(self):
          return [self[alias] for alias in self]
  
      def close_all(self):
          for alias in self:
              try:
                  connection = getattr(self._connections, alias)
              except AttributeError:
                  continue
              connection.close()
  
  
  class ConnectionRouter:
      def __init__(self, routers=None):
          """
          If routers is not specified, will default to settings.DATABASE_ROUTERS.
          """
          self._routers = routers
  
      @cached_property
      def routers(self):
          if self._routers is None:
              self._routers = settings.DATABASE_ROUTERS
          routers = []
          for r in self._routers:
              if isinstance(r, str):
                  router = import_string(r)()
              else:
                  router = r
              routers.append(router)
          return routers
  
      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
              instance = hints.get('instance')
              if instance is not None and instance._state.db:
                  return instance._state.db
              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_migrate(self, db, app_label, **hints):
          for router in self.routers:
              try:
                  method = router.allow_migrate
              except AttributeError:
                  # If the router doesn't have a method, skip to the next one.
                  continue
  
              allow = method(db, app_label, **hints)
  
              if allow is not None:
                  return allow
          return True
  
      def allow_migrate_model(self, db, model):
          return self.allow_migrate(
              db,
              model._meta.app_label,
              model_name=model._meta.model_name,
              model=model,
          )
  
      def get_migratable_models(self, app_config, db, include_auto_created=False):
          """
          Return app models allowed to be synchronized on provided db.
          """
          models = app_config.get_models(include_auto_created=include_auto_created)
          return [model for model in models if self.allow_migrate_model(db, model)]
  