self.__pool = PooledDB(creator=pymysql,
                       mincached=1, 
                       maxcached=4, # 連接池中最大空閑連接數(shù)
                       maxconnections=4,#允許的最大連接數(shù)
                       blocking=True,# 設(shè)置為true，則阻塞并等待直到連接數(shù)量減少，false默認(rèn)情況下將報(bào)告錯(cuò)誤。
                       ping=1,#默認(rèn)=1表示每當(dāng)從池中獲取時(shí)，使用ping()檢查連接
                       host=self.host,
                       port=self.port,
                       user=self.user,
                       passwd=self.passwd,
                       db=self.db_name,
                       charset=self.charset
                      )

# PooledDB.py源碼 267行
idle = [self.dedicated_connection() for i in range(mincached)]
while idle:
    idle.pop().close()
# 確實(shí)是創(chuàng)建連接池時(shí)創(chuàng)建了mincached個(gè)連接，但返回之前都關(guān)閉了。所以創(chuàng)建好的時(shí)候并沒有mincached個(gè)初始連接

# PooledDB.py源碼 255行
if maxconnections:
	if maxconnections < maxcached:
		maxconnections = maxcached
	if maxconnections < maxshared:
		maxconnections = maxshared
	self._maxconnections = maxconnections
else:
	self._maxconnections = 0
# maxcached、maxshared同時(shí)影響maxconnections
# maxconnections=max(maxcached, maxshared)

# PooledDB.py源碼 356行
	# 當(dāng)收到一個(gè)連接放回請求時(shí)
    # if 沒有最大空閑連接數(shù)限制，或現(xiàn)在的空閑連接數(shù)小于最大空閑連接數(shù)，則將事務(wù)回滾，并將這個(gè)連接放回空閑連接處；
    # else:直接關(guān)閉
    def cache(self, con):
        """Put a dedicated專用 connection back into the idle空閑 cache."""
        self._lock.acquire()
        try:
            if not self._maxcached or len(self._idle_cache) < self._maxcached:
                con._reset(force=self._reset)  # rollback possible transaction
                # the idle cache is not full, so put it there
                self._idle_cache.append(con)  # append it to the idle cache
            else:  # if the idle cache is already full,
                con.close()  # then close the connection
            self._connections -= 1
            self._lock.notify()
        finally:
            self._lock.release()
# cache方法被使用
    def close(self):
        """Close the pooled dedicated connection."""
        # Instead of actually closing the connection,
        # return it to the pool for future reuse.
        if self._con:
            self._pool.cache(self._con)
            self._con = None

# SteadyDB.py 483行
if self._maxusage:
	if self._usage >= self._maxusage:
        # the connection was used too often
        raise self._failure
cursor = self._con.cursor(*args, **kwargs)  # try to get a cursor

# SteadyDB.py 298行
    def _setsession(self, con=None):
        """Execute the SQL commands for session preparation."""
        if con is None:
            con = self._con
        if self._setsession_sql:
            cursor = con.cursor()
            for sql in self._setsession_sql:
                cursor.execute(sql)
            cursor.close()

    def cache(self, con):
        """Put a dedicated connection back into the idle cache."""
        self._lock.acquire()
        try:
            if not self._maxcached or len(self._idle_cache) < self._maxcached:
                con._reset(force=self._reset)  # rollback possible transaction
                # the idle cache is not full, so put it there
                self._idle_cache.append(con)  # append it to the idle cache
            else:  # if the idle cache is already full,
                con.close()  # then close the connection
            self._connections -= 1
            self._lock.notify()
        finally:
            self._lock.release()
 
    def _reset(self, force=False):
        """Reset a tough connection.

        Rollback if forced or the connection was in a transaction.

        """
        if not self._closed and (force or self._transaction):
            try:
                self.rollback()
            except Exception:
                pass
            
    def begin(self, *args, **kwargs):
        """Indicate the beginning of a transaction.

        During a transaction, connections won't be transparently
        replaced, and all errors will be raised to the application.

        If the underlying driver supports this method, it will be called
        with the given parameters (e.g. for distributed transactions).

        """
        self._transaction = True
        try:
            begin = self._con.begin
        except AttributeError:
            pass
        else:
            begin(*args, **kwargs)

except self._failures as error:

    def _ping_check(self, ping=1, reconnect=True):
        """Check whether the connection is still alive using ping().

        If the the underlying connection is not active and the ping
        parameter is set accordingly, the connection will be recreated
        unless the connection is currently inside a transaction.

        """
        if ping & self._ping:
            try:  # if possible, ping the connection
                alive = self._con.ping()
            except (AttributeError, IndexError, TypeError, ValueError):
                self._ping = 0  # ping() is not available
                alive = None
                reconnect = False
            except Exception:
                alive = False
            else:
                if alive is None:
                    alive = True
                if alive:
                    reconnect = False
            if reconnect and not self._transaction:
                try:  # try to reopen the connection
                    con = self._create()
                except Exception:
                    pass
                else:
                    self._close()
                    self._store(con)
                    alive = True
            return alive

    def start_conn(self):
        try:
            # maxshared 允許的最大共享連接數(shù)，默認(rèn)0/None表示所有連接都是專用的
            # 當(dāng)線程關(guān)閉不再共享的連接時(shí)，它將返回到空閑連接池中，以便可以再次對其進(jìn)行回收。
            # mincached 連接池中空閑連接的初始連接數(shù)，實(shí)驗(yàn)證明沒啥用
            self.__pool = PooledDB(creator=pymysql,
                                   mincached=1, # mincached 連接池中空閑連接的初始連接數(shù)，但其實(shí)沒用
                                   maxcached=4,  # 連接池中最大空閑連接數(shù)
                                   maxshared=3, #允許的最大共享連接數(shù)
                                   maxconnections=2,  # 允許的最大連接數(shù)
                                   blocking=False,  # 設(shè)置為true，則阻塞并等待直到連接數(shù)量減少，false默認(rèn)情況下將報(bào)告錯(cuò)誤。
                                   host=self.host,
                                   port=self.port,
                                   user=self.user,
                                   passwd=self.passwd,
                                   db=self.db_name,
                                   charset=self.charset
                                   )
            print("0 start_conn連接數(shù):%s " % (self.__pool._connections))
            self.conn = self.__pool.connection()
            print('connect success')
            print("1 start_conn連接數(shù):%s " % (self.__pool._connections))

            self.conn2 = self.__pool.connection()
            print("2 start_conn連接數(shù):%s " % (self.__pool._connections))
            db3 = self.__pool.connection()
            print("3 start_conn連接數(shù):%s " % (self.__pool._connections))
            db4 = self.__pool.connection()
            print("4 start_conn連接數(shù):%s " % (self.__pool._connections))
            db5 = self.__pool.connection()
            print("5 start_conn連接數(shù):%s " % (self.__pool._connections))
            # self.conn.close()
            print("6 start_conn連接數(shù):%s " % (self.__pool._connections))
            return True
        except:
            print('connect failed')
            return False