result.txt

$ python sample.py
...........................................................................................................................................................................................................................................................ERROR:sqlalchemy.pool.QueuePool:Exception closing connection None
Traceback (most recent call last):
  File "/Users/inada-n/venvs/flask-gevent-pymysql/lib/python2.7/site-packages/sqlalchemy/pool.py", line 244, in _close_connection
    self._dialect.do_close(connection)
  File "/Users/inada-n/venvs/flask-gevent-pymysql/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 411, in do_close
    dbapi_connection.close()
AttributeError: 'NoneType' object has no attribute 'close'
Traceback (most recent call last):
  File "/Users/inada-n/venvs/flask-gevent-pymysql/lib/python2.7/site-packages/gevent/greenlet.py", line 327, in run
    result = self._run(*self.args, **self.kwargs)
  File "sample.py", line 23, in worker
    Session.remove()
  File "/Users/inada-n/venvs/flask-gevent-pymysql/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 86, in remove
    self.registry().close()
  File "/Users/inada-n/venvs/flask-gevent-pymysql/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 999, in close
    transaction.close()
  File "/Users/inada-n/venvs/flask-gevent-pymysql/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 437, in close
    connection.close()
  File "/Users/inada-n/venvs/flask-gevent-pymysql/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 640, in close
    del self.__connection
AttributeError: _Connection__connection
<Greenlet at 0x10e62acd0: worker> failed with AttributeError

.....................................................................................................................................................................................................

sample.py

import gevent.monkey
gevent.monkey.patch_all()

import logging
logging.basicConfig()
import random
import sys
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker

engine = create_engine('mysql+pymysql://root@localhost/test',
                       pool_size=2, max_overflow=100, pool_recycle=5)
Session = scoped_session(sessionmaker(bind=engine))

stop = False

def worker():
    global stop
    try:
        while True:
            Session.execute("SELECT 1+1")
            gevent.sleep(random.random()*10)
            Session.remove()
            sys.stderr.write('.')
    finally:
        stop = True


def main():
    for i in range(100):
        gevent.spawn(worker)

    gevent.sleep(10)

    result = list(engine.execute("show processlist"))
    engine.execute("kill %d" % result[-2][0])

    while not stop:
        gevent.sleep(1)
    gevent.sleep(10)


main()