class PooledDedicatedDBConnection:
    """Auxiliary proxy class for pooled dedicated connections."""

    def __init__(self, pool, con):
        """Create a pooled dedicated connection.

        pool: the corresponding PooledDB instance
        con: the underlying SteadyDB connection
        """
        # basic initialization to make finalizer work
        self._con = None
        # proper initialization of the connection
        if not con.threadsafety():
            raise NotSupportedError("Database module is not thread-safe.")
        self._pool = pool
        self._con = con

    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

    def __getattr__(self, name):
        """Proxy all members of the class."""
        if self._con:
            return getattr(self._con, name)
        else:
            raise InvalidConnection

    def __del__(self):
        """Delete the pooled connection."""
        try:
            self.close()
        except:  # builtin Exceptions might not exist any more
            pass

    def __enter__(self):
        """Enter a runtime context for the connection."""
        return self

    def __exit__(self, *exc):
        """Exit a runtime context for the connection."""
        self.close()

class PooledDB:
    """Pool for DB-API 2 connections.

    After you have created the connection pool, you can use
    connection() to get pooled, steady DB-API 2 connections.
    """

    version = __version__

    def __init__(
            self, creator, mincached=0, maxcached=0,
            maxshared=0, maxconnections=0, blocking=False,
            maxusage=None, setsession=None, reset=True,
            failures=None, ping=1,
            *args, **kwargs):
        """Set up the DB-API 2 connection pool.

        creator: either an arbitrary function returning new DB-API 2
            connection objects or a DB-API 2 compliant database module
        mincached: initial number of idle connections in the pool
            (0 means no connections are made at startup)
        maxcached: maximum number of idle connections in the pool
            (0 or None means unlimited pool size)
        maxshared: maximum number of shared connections
            (0 or None means all connections are dedicated)
            When this maximum number is reached, connections are
            shared if they have been requested as shareable.
        maxconnections: maximum number of connections generally allowed
            (0 or None means an arbitrary number of connections)
        blocking: determines behavior when exceeding the maximum
            (if this is set to true, block and wait until the number of
            connections decreases, otherwise an error will be reported)
        maxusage: maximum number of reuses of a single connection
            (0 or None means unlimited reuse)
            When this maximum usage number of the connection is reached,
            the connection is automatically reset (closed and reopened).
        setsession: optional list of SQL commands that may serve to prepare
            the session, e.g. ["set datestyle to ...", "set time zone ..."]
        reset: how connections should be reset when returned to the pool
            (False or None to rollback transcations started with begin(),
            True to always issue a rollback for safety's sake)
        failures: an optional exception class or a tuple of exception classes
            for which the connection failover mechanism shall be applied,
            if the default (OperationalError, InternalError) is not adequate
        ping: determines when the connection should be checked with ping()
            (0 = None = never, 1 = default = whenever fetched from the pool,
            2 = when a cursor is created, 4 = when a query is executed,
            7 = always, and all other bit combinations of these values)
        args, kwargs: the parameters that shall be passed to the creator
            function or the connection constructor of the DB-API 2 module
        """
        try:
            threadsafety = creator.threadsafety
        except AttributeError:
            try:
                if not callable(creator.connect):
                    raise AttributeError
            except AttributeError:
                threadsafety = 2
            else:
                threadsafety = 0
        if not threadsafety:
            raise NotSupportedError("Database module is not thread-safe.")
        self._creator = creator
        self._args, self._kwargs = args, kwargs
        self._blocking = blocking
        self._maxusage = maxusage
        self._setsession = setsession
        self._reset = reset
        self._failures = failures
        self._ping = ping
        if mincached is None:
            mincached = 0
        if maxcached is None:
            maxcached = 0
        if maxconnections is None:
            maxconnections = 0
        if maxcached:
            if maxcached < mincached:
                maxcached = mincached
            self._maxcached = maxcached
        else:
            self._maxcached = 0
        if threadsafety > 1 and maxshared:
            self._maxshared = maxshared
            self._shared_cache = []  # the cache for shared connections
        else:
            self._maxshared = 0
        if maxconnections: # 设置了最大允许连接数
            if maxconnections < maxcached: # 最大允许连接数 小于 pool中最大空闲连接数
                maxconnections = maxcached
            if maxconnections < maxshared:
                maxconnections = maxshared
            self._maxconnections = maxconnections
        else:
            self._maxconnections = 0
        self._idle_cache = []  # the actual pool of idle connections
        self._lock = Condition()
        self._connections = 0
        # Establish an initial number of idle database connections:
        idle = [self.dedicated_connection() for i in range(mincached)]
        while idle:
            idle.pop().close()

    def steady_connection(self):
        """Get a steady, unpooled DB-API 2 connection."""
        return connect(
            self._creator, self._maxusage, self._setsession,
            self._failures, self._ping, True, *self._args, **self._kwargs)

    def connection(self, shareable=True):
        """Get a steady, cached DB-API 2 connection from the pool.

        If shareable is set and the underlying DB-API 2 allows it,
        then the connection may be shared with other threads.
        """
        if shareable and self._maxshared:
            with self._lock:
                # 保证线程安全
                while (not self._shared_cache and self._maxconnections
                        and self._connections >= self._maxconnections):
                    self._wait_lock()
                if len(self._shared_cache) < self._maxshared:
                    # shared cache is not full, get a dedicated connection
                    try:  # first try to get it from the idle cache
                        con = self._idle_cache.pop(0)
                    except IndexError:  # else get a fresh connection
                        con = self.steady_connection()
                    else:
                        con._ping_check()  # check this connection
                    con = SharedDBConnection(con)
                    self._connections += 1
                else:  # shared cache full or no more connections allowed
                    self._shared_cache.sort()  # least shared connection first
                    con = self._shared_cache.pop(0)  # get it
                    while con.con._transaction:
                        # do not share connections which are in a transaction
                        self._shared_cache.insert(0, con)
                        self._wait_lock()
                        self._shared_cache.sort()
                        con = self._shared_cache.pop(0)
                    con.con._ping_check()  # check the underlying connection
                    con.share()  # increase share of this connection
                # put the connection (back) into the shared cache
                self._shared_cache.append(con)
                self._lock.notify()
            con = PooledSharedDBConnection(self, con)
        else:  # try to get a dedicated connection
            with self._lock:
                while (self._maxconnections
                        and self._connections >= self._maxconnections):
                    self._wait_lock()
                # connection limit not reached, get a dedicated connection
                try:  # first try to get it from the idle cache
                    con = self._idle_cache.pop(0)
                except IndexError:  # else get a fresh connection
                    con = self.steady_connection()
                else:
                    con._ping_check()  # check connection
                con = PooledDedicatedDBConnection(self, con)
                self._connections += 1
        return con