text: # sqlalchemy/pool.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
  
  
  """Connection pooling for DB-API connections.
  
  Provides a number of connection pool implementations for a variety of
  usage scenarios and thread behavior requirements imposed by the
  application, DB-API or database itself.
  
  Also provides a DB-API 2.0 connection proxying mechanism allowing
  regular DB-API connect() methods to be transparently managed by a
  SQLAlchemy connection pool.
  """
  
  import weakref, time, traceback
  
  from sqlalchemy import exc, log, event, events, interfaces, util
  from sqlalchemy.util import queue as sqla_queue
  from sqlalchemy.util import threading, memoized_property, \
      chop_traceback
  
  proxies = {}
  
  def manage(module, **params):
      """Return a proxy for a DB-API module that automatically 
      pools connections.
  
      Given a DB-API 2.0 module and pool management parameters, returns
      a proxy for the module that will automatically pool connections,
      creating new connection pools for each distinct set of connection
      arguments sent to the decorated module's connect() function.
  
      :param module: a DB-API 2.0 database module
  
      :param poolclass: the class used by the pool module to provide
        pooling.  Defaults to :class:`.QueuePool`.
  
      :param \*\*params: will be passed through to *poolclass*
  
      """
      try:
          return proxies[module]
      except KeyError:
          return proxies.setdefault(module, _DBProxy(module, **params))
  
  def clear_managers():
      """Remove all current DB-API 2.0 managers.
  
      All pools and connections are disposed.
      """
  
      for manager in proxies.itervalues():
          manager.close()
      proxies.clear()
  
  
  class Pool(log.Identified):
      """Abstract base class for connection pools."""
  
      def __init__(self, 
                      creator, recycle=-1, echo=None, 
                      use_threadlocal=False,
                      logging_name=None,
                      reset_on_return=True, 
                      listeners=None,
                      events=None,
                      _dispatch=None):
          """
          Construct a Pool.
  
          :param creator: a callable function that returns a DB-API
            connection object.  The function will be called with
            parameters.
  
          :param recycle: If set to non -1, number of seconds between
            connection recycling, which means upon checkout, if this
            timeout is surpassed the connection will be closed and
            replaced with a newly opened connection. Defaults to -1.
  
          :param logging_name:  String identifier which will be used within
            the "name" field of logging records generated within the 
            "sqlalchemy.pool" logger. Defaults to a hexstring of the object's 
            id.
  
          :param echo: If True, connections being pulled and retrieved
            from the pool will be logged to the standard output, as well
            as pool sizing information.  Echoing can also be achieved by
            enabling logging for the "sqlalchemy.pool"
            namespace. Defaults to False.
  
          :param use_threadlocal: If set to True, repeated calls to
            :meth:`connect` within the same application thread will be
            guaranteed to return the same connection object, if one has
            already been retrieved from the pool and has not been
            returned yet.  Offers a slight performance advantage at the
            cost of individual transactions by default.  The
            :meth:`unique_connection` method is provided to bypass the
            threadlocal behavior installed into :meth:`connect`.
  
          :param reset_on_return: If true, reset the database state of
            connections returned to the pool.  This is typically a
            ROLLBACK to release locks and transaction resources.
            Disable at your own peril.  Defaults to True.
  
          :param events: a list of 2-tuples, each of the form
           ``(callable, target)`` which will be passed to event.listen()
           upon construction.   Provided here so that event listeners
           can be assigned via ``create_engine`` before dialect-level
           listeners are applied.
  
          :param listeners: Deprecated.  A list of
            :class:`~sqlalchemy.interfaces.PoolListener`-like objects or
            dictionaries of callables that receive events when DB-API
            connections are created, checked out and checked in to the
            pool.  This has been superseded by 
            :func:`~sqlalchemy.event.listen`.
  
          """
          if logging_name:
              self.logging_name = self._orig_logging_name = logging_name
          else:
              self._orig_logging_name = None
  
          log.instance_logger(self, echoflag=echo)
          self._threadconns = threading.local()
          self._creator = creator
          self._recycle = recycle
          self._use_threadlocal = use_threadlocal
          self._reset_on_return = reset_on_return
          self.echo = echo
          if _dispatch:
              self.dispatch._update(_dispatch, only_propagate=False)
          if events:
              for fn, target in events:
                  event.listen(self, target, fn)
          if listeners:
              util.warn_deprecated(
                          "The 'listeners' argument to Pool (and "
                          "create_engine()) is deprecated.  Use event.listen().")
              for l in listeners:
                  self.add_listener(l)
  
      dispatch = event.dispatcher(events.PoolEvents)
  
      @util.deprecated(2.7, "Pool.add_listener is deprecated.  Use event.listen()")
      def add_listener(self, listener):
          """Add a :class:`.PoolListener`-like object to this pool.
  
          ``listener`` may be an object that implements some or all of
          PoolListener, or a dictionary of callables containing implementations
          of some or all of the named methods in PoolListener.
  
          """
          interfaces.PoolListener._adapt_listener(self, listener)
  
      def unique_connection(self):
          """Produce a DBAPI connection that is not referenced by any
          thread-local context.
  
          This method is different from :meth:`.Pool.connect` only if the
          ``use_threadlocal`` flag has been set to ``True``.
  
          """
  
          return _ConnectionFairy(self).checkout()
  
      def _create_connection(self):
          """Called by subclasses to create a new ConnectionRecord."""
  
          return _ConnectionRecord(self)
  
      def recreate(self):
          """Return a new :class:`.Pool`, of the same class as this one
          and configured with identical creation arguments.
  
          This method is used in conjunection with :meth:`dispose` 
          to close out an entire :class:`.Pool` and create a new one in 
          its place.
  
          """
  
          raise NotImplementedError()
  
      def dispose(self):
          """Dispose of this pool.
  
          This method leaves the possibility of checked-out connections
          remaining open, It is advised to not reuse the pool once dispose()
          is called, and to instead use a new pool constructed by the
          recreate() method.
  
          """
  
          raise NotImplementedError()
  
      def connect(self):
          """Return a DBAPI connection from the pool.
  
          The connection is instrumented such that when its 
          ``close()`` method is called, the connection will be returned to 
          the pool.
  
          """
          if not self._use_threadlocal:
              return _ConnectionFairy(self).checkout()
  
          try:
              rec = self._threadconns.current()
              if rec:
                  return rec.checkout()
          except AttributeError:
              pass
  
          agent = _ConnectionFairy(self)
          self._threadconns.current = weakref.ref(agent)
          return agent.checkout()
  
      def _return_conn(self, record):
          """Given a _ConnectionRecord, return it to the :class:`.Pool`.
  
          This method is called when an instrumented DBAPI connection
          has its ``close()`` method called.
  
          """
          if self._use_threadlocal:
              try:
                  del self._threadconns.current
              except AttributeError:
                  pass
          self._do_return_conn(record)
  
      def _do_get(self):
          """Implementation for :meth:`get`, supplied by subclasses."""
  
          raise NotImplementedError()
  
      def _do_return_conn(self, conn):
          """Implementation for :meth:`return_conn`, supplied by subclasses."""
  
          raise NotImplementedError()
  
      def status(self):
          raise NotImplementedError()
  
  
  class _ConnectionRecord(object):
      finalize_callback = None
  
      def __init__(self, pool):
          self.__pool = pool
          self.connection = self.__connect()
          self.info = {}
  
          pool.dispatch.first_connect.exec_once(self.connection, self)
          pool.dispatch.connect(self.connection, self)
  
      def close(self):
          if self.connection is not None:
              self.__pool.logger.debug("Closing connection %r", self.connection)
              try:
                  self.connection.close()
              except (SystemExit, KeyboardInterrupt):
                  raise
              except:
                  self.__pool.logger.debug("Exception closing connection %r",
                                  self.connection)
  
      def invalidate(self, e=None):
          if e is not None:
              self.__pool.logger.info(
                  "Invalidate connection %r (reason: %s:%s)",
                  self.connection, e.__class__.__name__, e)
          else:
              self.__pool.logger.info(
                  "Invalidate connection %r", self.connection)
          self.__close()
          self.connection = None
  
      def get_connection(self):
          if self.connection is None:
              self.connection = self.__connect()
              self.info.clear()
              if self.__pool.dispatch.connect:
                  self.__pool.dispatch.connect(self.connection, self)
          elif self.__pool._recycle > -1 and \
                  time.time() - self.starttime > self.__pool._recycle:
              self.__pool.logger.info(
                      "Connection %r exceeded timeout; recycling",
                      self.connection)
              self.__close()
              self.connection = self.__connect()
              self.info.clear()
              if self.__pool.dispatch.connect:
                  self.__pool.dispatch.connect(self.connection, self)
          return self.connection
  
      def __close(self):
          try:
              self.__pool.logger.debug("Closing connection %r", self.connection)
              self.connection.close()
          except (SystemExit, KeyboardInterrupt):
              raise
          except Exception, e:
              self.__pool.logger.debug(
                          "Connection %r threw an error on close: %s",
                          self.connection, e)
  
      def __connect(self):
          try:
              self.starttime = time.time()
              connection = self.__pool._creator()
              self.__pool.logger.debug("Created new connection %r", connection)
              return connection
          except Exception, e:
              self.__pool.logger.debug("Error on connect(): %s", e)
              raise
  
  
  def _finalize_fairy(connection, connection_record, pool, ref, echo):
      _refs.discard(connection_record)
  
      if ref is not None and \
                  connection_record.fairy is not ref:
          return
  
      if connection is not None:
          try:
              if pool._reset_on_return:
                  connection.rollback()
              # Immediately close detached instances
              if connection_record is None:
                  connection.close()
          except Exception, e:
              if connection_record is not None:
                  connection_record.invalidate(e=e)
              if isinstance(e, (SystemExit, KeyboardInterrupt)):
                  raise
  
      if connection_record is not None:
          connection_record.fairy = None
          if echo:
              pool.logger.debug("Connection %r being returned to pool", 
                                      connection)
          if connection_record.finalize_callback:
              connection_record.finalize_callback(connection)
              del connection_record.finalize_callback 
          if pool.dispatch.checkin:
              pool.dispatch.checkin(connection, connection_record)
          pool._return_conn(connection_record)
  
  _refs = set()
  
  class _ConnectionFairy(object):
      """Proxies a DB-API connection and provides return-on-dereference
      support."""
  
      __slots__ = '_pool', '__counter', 'connection', \
                  '_connection_record', '__weakref__', \
                  '_detached_info', '_echo'
  
      def __init__(self, pool):
          self._pool = pool
          self.__counter = 0
          self._echo = _echo = pool._should_log_debug()
          try:
              rec = self._connection_record = pool._do_get()
              conn = self.connection = self._connection_record.get_connection()
              rec.fairy = weakref.ref(
                              self, 
                              lambda ref:_finalize_fairy(conn, rec, pool, ref, _echo)
                          )
              _refs.add(rec)
          except:
              # helps with endless __getattr__ loops later on
              self.connection = None 
              self._connection_record = None
              raise
          if self._echo:
              self._pool.logger.debug("Connection %r checked out from pool" %
                         self.connection)
  
      @property
      def _logger(self):
          return self._pool.logger
  
      @property
      def is_valid(self):
          return self.connection is not None
  
      @property
      def info(self):
          """An info collection unique to this DB-API connection."""
  
          try:
              return self._connection_record.info
          except AttributeError:
              if self.connection is None:
                  raise exc.InvalidRequestError("This connection is closed")
              try:
                  return self._detached_info
              except AttributeError:
                  self._detached_info = value = {}
                  return value
  
      def invalidate(self, e=None):
          """Mark this connection as invalidated.
  
          The connection will be immediately closed.  The containing
          ConnectionRecord will create a new connection when next used.
          """
  
          if self.connection is None:
              raise exc.InvalidRequestError("This connection is closed")
          if self._connection_record is not None:
              self._connection_record.invalidate(e=e)
          self.connection = None
          self._close()
  
      def cursor(self, *args, **kwargs):
          return self.connection.cursor(*args, **kwargs)
  
      def __getattr__(self, key):
          return getattr(self.connection, key)
  
      def checkout(self):
          if self.connection is None:
              raise exc.InvalidRequestError("This connection is closed")
          self.__counter += 1
  
          if not self._pool.dispatch.checkout or self.__counter != 1:
              return self
  
          # Pool listeners can trigger a reconnection on checkout
          attempts = 2
          while attempts > 0:
              try:
                  self._pool.dispatch.checkout(self.connection, 
                                              self._connection_record,
                                              self)
                  return self
              except exc.DisconnectionError, e:
                  self._pool.logger.info(
                  "Disconnection detected on checkout: %s", e)
                  self._connection_record.invalidate(e)
                  self.connection = self._connection_record.get_connection()
                  attempts -= 1
  
          self._pool.logger.info("Reconnection attempts exhausted on checkout")
          self.invalidate()
          raise exc.InvalidRequestError("This connection is closed")
  
      def detach(self):
          """Separate this connection from its Pool.
  
          This means that the connection will no longer be returned to the
          pool when closed, and will instead be literally closed.  The
          containing ConnectionRecord is separated from the DB-API connection,
          and will create a new connection when next used.
  
          Note that any overall connection limiting constraints imposed by a
          Pool implementation may be violated after a detach, as the detached
          connection is removed from the pool's knowledge and control.
          """
  
          if self._connection_record is not None:
              _refs.remove(self._connection_record)
              self._connection_record.fairy = None
              self._connection_record.connection = None
              self._pool._do_return_conn(self._connection_record)
              self._detached_info = \
                self._connection_record.info.copy()
              self._connection_record = None
  
      def close(self):
          self.__counter -= 1
          if self.__counter == 0:
              self._close()
  
      def _close(self):
          _finalize_fairy(self.connection, self._connection_record, 
                              self._pool, None, self._echo)
          self.connection = None
          self._connection_record = None
  
  class SingletonThreadPool(Pool):
      """A Pool that maintains one connection per thread.
  
      Maintains one connection per each thread, never moving a connection to a
      thread other than the one which it was created in.
  
      Options are the same as those of :class:`.Pool`, as well as:
  
      :param pool_size: The number of threads in which to maintain connections 
          at once.  Defaults to five.
  
      :class:`.SingletonThreadPool` is used by the SQLite dialect
      automatically when a memory-based database is used.
      See :ref:`sqlite_toplevel`.
  
      """
  
      def __init__(self, creator, pool_size=5, **kw):
          kw['use_threadlocal'] = True
          Pool.__init__(self, creator, **kw)
          self._conn = threading.local()
          self._all_conns = set()
          self.size = pool_size
  
      def recreate(self):
          self.logger.info("Pool recreating")
          return SingletonThreadPool(self._creator, 
              pool_size=self.size, 
              recycle=self._recycle, 
              echo=self.echo, 
              logging_name=self._orig_logging_name,
              use_threadlocal=self._use_threadlocal, 
              _dispatch=self.dispatch)
  
      def dispose(self):
          """Dispose of this pool."""
  
          for conn in self._all_conns:
              try:
                  conn.close()
              except (SystemExit, KeyboardInterrupt):
                  raise
              except:
                  # pysqlite won't even let you close a conn from a thread
                  # that didn't create it
                  pass
  
          self._all_conns.clear()
  
      def _cleanup(self):
          while len(self._all_conns) > self.size:
              c = self._all_conns.pop()
              c.close()
  
      def status(self):
          return "SingletonThreadPool id:%d size: %d" % \
                              (id(self), len(self._all_conns))
  
      def _do_return_conn(self, conn):
          pass
  
      def _do_get(self):
          try:
              c = self._conn.current()
              if c:
                  return c
          except AttributeError:
              pass
          c = self._create_connection()
          self._conn.current = weakref.ref(c)
          self._all_conns.add(c)
          if len(self._all_conns) > self.size:
              self._cleanup()
          return c
  
  class QueuePool(Pool):
      """A :class:`.Pool` that imposes a limit on the number of open connections.
  
      :class:`.QueuePool` is the default pooling implementation used for 
      all :class:`.Engine` objects, unless the SQLite dialect is in use.
  
      """
  
      def __init__(self, creator, pool_size=5, max_overflow=10, timeout=30,
                   **kw):
          """
          Construct a QueuePool.
  
          :param creator: a callable function that returns a DB-API
            connection object.  The function will be called with
            parameters.
  
          :param pool_size: The size of the pool to be maintained,
            defaults to 5. This is the largest number of connections that
            will be kept persistently in the pool. Note that the pool
            begins with no connections; once this number of connections
            is requested, that number of connections will remain.
            ``pool_size`` can be set to 0 to indicate no size limit; to
            disable pooling, use a :class:`~sqlalchemy.pool.NullPool`
            instead.
  
          :param max_overflow: The maximum overflow size of the
            pool. When the number of checked-out connections reaches the
            size set in pool_size, additional connections will be
            returned up to this limit. When those additional connections
            are returned to the pool, they are disconnected and
            discarded. It follows then that the total number of
            simultaneous connections the pool will allow is pool_size +
            `max_overflow`, and the total number of "sleeping"
            connections the pool will allow is pool_size. `max_overflow`
            can be set to -1 to indicate no overflow limit; no limit
            will be placed on the total number of concurrent
            connections. Defaults to 10.
  
          :param timeout: The number of seconds to wait before giving up
            on returning a connection. Defaults to 30.
  
          :param recycle: If set to non -1, number of seconds between
            connection recycling, which means upon checkout, if this
            timeout is surpassed the connection will be closed and
            replaced with a newly opened connection. Defaults to -1.
  
          :param echo: If True, connections being pulled and retrieved
            from the pool will be logged to the standard output, as well
            as pool sizing information.  Echoing can also be achieved by
            enabling logging for the "sqlalchemy.pool"
            namespace. Defaults to False.
  
          :param use_threadlocal: If set to True, repeated calls to
            :meth:`connect` within the same application thread will be
            guaranteed to return the same connection object, if one has
            already been retrieved from the pool and has not been
            returned yet.  Offers a slight performance advantage at the
            cost of individual transactions by default.  The
            :meth:`unique_connection` method is provided to bypass the
            threadlocal behavior installed into :meth:`connect`.
  
          :param reset_on_return: If true, reset the database state of
            connections returned to the pool.  This is typically a
            ROLLBACK to release locks and transaction resources.
            Disable at your own peril.  Defaults to True.
  
          :param listeners: A list of
            :class:`~sqlalchemy.interfaces.PoolListener`-like objects or
            dictionaries of callables that receive events when DB-API
            connections are created, checked out and checked in to the
            pool.
  
          """
          Pool.__init__(self, creator, **kw)
          self._pool = sqla_queue.Queue(pool_size)
          self._overflow = 0 - pool_size
          self._max_overflow = max_overflow
          self._timeout = timeout
          self._overflow_lock = self._max_overflow > -1 and \
                                      threading.Lock() or None
  
      def recreate(self):
          self.logger.info("Pool recreating")
          return QueuePool(self._creator, pool_size=self._pool.maxsize, 
                            max_overflow=self._max_overflow,
                            timeout=self._timeout, 
                            recycle=self._recycle, echo=self.echo, 
                            logging_name=self._orig_logging_name,
                            use_threadlocal=self._use_threadlocal,
                            _dispatch=self.dispatch)
  
      def _do_return_conn(self, conn):
          try:
              self._pool.put(conn, False)
          except sqla_queue.Full:
              conn.close()
              if self._overflow_lock is None:
                  self._overflow -= 1
              else:
                  self._overflow_lock.acquire()
                  try:
                      self._overflow -= 1
                  finally:
                      self._overflow_lock.release()
  
      def _do_get(self):
          try:
              wait = self._max_overflow > -1 and \
                          self._overflow >= self._max_overflow
              return self._pool.get(wait, self._timeout)
          except sqla_queue.Empty:
              if self._max_overflow > -1 and \
                          self._overflow >= self._max_overflow:
                  if not wait:
                      return self._do_get()
                  else:
                      raise exc.TimeoutError(
                              "QueuePool limit of size %d overflow %d reached, "
                              "connection timed out, timeout %d" % 
                              (self.size(), self.overflow(), self._timeout))
  
              if self._overflow_lock is not None:
                  self._overflow_lock.acquire()
  
              if self._max_overflow > -1 and \
                          self._overflow >= self._max_overflow:
                  if self._overflow_lock is not None:
                      self._overflow_lock.release()
                  return self._do_get()
  
              try:
                  con = self._create_connection()
                  self._overflow += 1
              finally:
                  if self._overflow_lock is not None:
                      self._overflow_lock.release()
              return con
  
      def dispose(self):
          while True:
              try:
                  conn = self._pool.get(False)
                  conn.close()
              except sqla_queue.Empty:
                  break
  
          self._overflow = 0 - self.size()
          self.logger.info("Pool disposed. %s", self.status())
  
      def status(self):
          return "Pool size: %d  Connections in pool: %d "\
                  "Current Overflow: %d Current Checked out "\
                  "connections: %d" % (self.size(), 
                                      self.checkedin(), 
                                      self.overflow(), 
                                      self.checkedout())
  
      def size(self):
          return self._pool.maxsize
  
      def checkedin(self):
          return self._pool.qsize()
  
      def overflow(self):
          return self._overflow
  
      def checkedout(self):
          return self._pool.maxsize - self._pool.qsize() + self._overflow
  
  class NullPool(Pool):
      """A Pool which does not pool connections.
  
      Instead it literally opens and closes the underlying DB-API connection
      per each connection open/close.
  
      Reconnect-related functions such as ``recycle`` and connection
      invalidation are not supported by this Pool implementation, since
      no connections are held persistently.
  
      :class:`.NullPool` is used by the SQlite dilalect automatically
      when a file-based database is used (as of SQLAlchemy 0.7).
      See :ref:`sqlite_toplevel`.
  
      """
  
      def status(self):
          return "NullPool"
  
      def _do_return_conn(self, conn):
          conn.close()
  
      def _do_get(self):
          return self._create_connection()
  
      def recreate(self):
          self.logger.info("Pool recreating")
  
          return NullPool(self._creator, 
              recycle=self._recycle, 
              echo=self.echo, 
              logging_name=self._orig_logging_name,
              use_threadlocal=self._use_threadlocal, 
              _dispatch=self.dispatch)
  
      def dispose(self):
          pass
  
  
  class StaticPool(Pool):
      """A Pool of exactly one connection, used for all requests.
  
      Reconnect-related functions such as ``recycle`` and connection
      invalidation (which is also used to support auto-reconnect) are not
      currently supported by this Pool implementation but may be implemented
      in a future release.
  
      """
  
      @memoized_property
      def _conn(self):
          return self._creator()
  
      @memoized_property
      def connection(self):
          return _ConnectionRecord(self)
  
      def status(self):
          return "StaticPool"
  
      def dispose(self):
          if '_conn' in self.__dict__:
              self._conn.close()
              self._conn = None
  
      def recreate(self):
          self.logger.info("Pool recreating")
          return self.__class__(creator=self._creator,
                                recycle=self._recycle,
                                use_threadlocal=self._use_threadlocal,
                                reset_on_return=self._reset_on_return,
                                echo=self.echo,
                                logging_name=self._orig_logging_name,
                                _dispatch=self.dispatch)
  
      def _create_connection(self):
          return self._conn
  
      def _do_return_conn(self, conn):
          pass
  
      def _do_get(self):
          return self.connection
  
  class AssertionPool(Pool):
      """A :class:`.Pool` that allows at most one checked out connection at any given
      time.
  
      This will raise an exception if more than one connection is checked out
      at a time.  Useful for debugging code that is using more connections
      than desired.
      
      :class:`.AssertionPool` also logs a traceback of where
      the original connection was checked out, and reports
      this in the assertion error raised (new in 0.7).
  
      """
      def __init__(self, *args, **kw):
          self._conn = None
          self._checked_out = False
          self._store_traceback = kw.pop('store_traceback', True)
          self._checkout_traceback = None
          Pool.__init__(self, *args, **kw)
  
      def status(self):
          return "AssertionPool"
  
      def _do_return_conn(self, conn):
          if not self._checked_out:
              raise AssertionError("connection is not checked out")
          self._checked_out = False
          assert conn is self._conn
  
      def dispose(self):
          self._checked_out = False
          if self._conn:
              self._conn.close()
  
      def recreate(self):
          self.logger.info("Pool recreating")
          return AssertionPool(self._creator, echo=self.echo, 
                              logging_name=self._orig_logging_name,
                              _dispatch=self.dispatch)
  
      def _do_get(self):
          if self._checked_out:
              if self._checkout_traceback:
                  suffix = ' at:\n%s' % ''.join(
                      chop_traceback(self._checkout_traceback))
              else:
                  suffix = ''
              raise AssertionError("connection is already checked out" + suffix)
  
          if not self._conn:
              self._conn = self._create_connection()
  
          self._checked_out = True
          if self._store_traceback:
              self._checkout_traceback = traceback.format_stack()
          return self._conn
  
  class _DBProxy(object):
      """Layers connection pooling behavior on top of a standard DB-API module.
  
      Proxies a DB-API 2.0 connect() call to a connection pool keyed to the
      specific connect parameters. Other functions and attributes are delegated
      to the underlying DB-API module.
      """
  
      def __init__(self, module, poolclass=QueuePool, **kw):
          """Initializes a new proxy.
  
          module
            a DB-API 2.0 module
  
          poolclass
            a Pool class, defaulting to QueuePool
  
          Other parameters are sent to the Pool object's constructor.
  
          """
  
          self.module = module
          self.kw = kw
          self.poolclass = poolclass
          self.pools = {}
          self._create_pool_mutex = threading.Lock()
  
      def close(self):
          for key in self.pools.keys():
              del self.pools[key]
  
      def __del__(self):
          self.close()
  
      def __getattr__(self, key):
          return getattr(self.module, key)
  
      def get_pool(self, *args, **kw):
          key = self._serialize(*args, **kw)
          try:
              return self.pools[key]
          except KeyError:
              self._create_pool_mutex.acquire()
              try:
                  if key not in self.pools:
                      pool = self.poolclass(lambda: 
                                  self.module.connect(*args, **kw), **self.kw)
                      self.pools[key] = pool
                      return pool
                  else:
                      return self.pools[key]
              finally:
                  self._create_pool_mutex.release()
  
      def connect(self, *args, **kw):
          """Activate a connection to the database.
  
          Connect to the database using this DBProxy's module and the given
          connect arguments.  If the arguments match an existing pool, the
          connection will be returned from the pool's current thread-local
          connection instance, or if there is no thread-local connection
          instance it will be checked out from the set of pooled connections.
  
          If the pool has no available connections and allows new connections
          to be created, a new database connection will be made.
  
          """
  
          return self.get_pool(*args, **kw).connect()
  
      def dispose(self, *args, **kw):
          """Dispose the pool referenced by the given connect arguments."""
  
          key = self._serialize(*args, **kw)
          try:
              del self.pools[key]
          except KeyError:
              pass
  
      def _serialize(self, *args, **kw):
          return tuple(
              list(args) + 
              [(k, kw[k]) for k in sorted(kw)]
          )
  