how can I release mongodb connections?

I have a mongo server with high read/write in a short time. I used python and pymongo, when I wake up this morning I found no connection can make to mongod master cause it's connections reached 19992, its a pretty much scary number

even I stopped all the program, the connection number seems no change

insert  query update delete getmore command flushes mapped  vsize    res faults locked % idx miss %     qr|qw   ar|aw  netIn netOut  conn repl       time

localhost:27417      0      0      0      0       2       1       0   624m  2.87g   287m      0        0          0       0|0     7|0   162b     1k 19992    M   10:36:16

> db.serverStatus(1)
{
        "host" : "42yl:27417",
        "version" : "1.8.1",
        "process" : "mongod",
        "uptime" : 71732,
        "uptimeEstimate" : 71470,
        "localTime" : ISODate("2011-05-26T03:02:48.301Z"),
        "globalLock" : {
                "totalTime" : 71732232290,
                "lockTime" : 149471421,
                "ratio" : 0.002083741384148133,
                "currentQueue" : {
                        "total" : 0,
                        "readers" : 0,
                        "writers" : 0
                },
                "activeClients" : {
                        "total" : 7,
                        "readers" : 7,
                        "writers" : 0
                }
        },
        "mem" : {
                "bits" : 32,
                "resident" : 258,
                "virtual" : 910,
                "supported" : true,
                "mapped" : 624
        },
        "connections" : {
                "current" : 19792,
                "available" : 208
        },
        "extra_info" : {
                "note" : "fields vary by platform",
                "heap_usage_bytes" : 562688,
                "page_faults" : 965
        },
        "indexCounters" : {
                "btree" : {
                        "accesses" : 12789,
                        "hits" : 12789,