Python Node.js到前端数据发送失败

我希望Node.js get请求将数据从python发送到前端。res.send只被调用过一次。但是，process.stdout.on会被多次调用，从而导致出现“无法设置头”错误。我希望服务器代码一次发送整个数据，而不需要多次调用process.stdout。我做错了什么

我试着使代码同步。但是，它不起作用。此外，我还尝试使用if头未发送的if条件调用process.stdout.on函数一次。这也不允许将python代码的整个结果发送到d3.js中的前端

app.get('/fetch_slider_val/:sliderVal',(req,res)=>{
    var sliderVal = req.params.sliderVal
    var X = sliderVal.split(" ")
    var sites = X[0].split(',')
    var date1 = X[1]
    var date2 = X[2]
    var normKey = X[3]
    var normEnt = X[4]

    var spawn = require('child_process').spawn;
    var process = spawn('python',['sql_data_retrieve.py',sites,date1,date2,normKey,normEnt]);
    console.log('1 out')
    i = 0
    process.stdout.on('data',function(data){
    console.log(res.headersSent) #false and becomes true after the next console.log(res.headersSent) gives true. somehow, this function is called again. 
        res.send(data.toString()) #some data sent not all 
        console.log(res.headersSent) #true
    })
})

我希望将整个data.toString（）发送到前端。然而，它只发送了一半，最后我收到了这个错误消息

_http_outgoing.js:470
    throw new ERR_HTTP_HEADERS_SENT('set');
    ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:470:11)
    at ServerResponse.header (/Users/amishr45/PycharmProjects/NewsVis/TrialVis/node_modules/express/lib/response.js:771:10)
    at ServerResponse.send (/Users/amishr45/PycharmProjects/NewsVis/TrialVis/node_modules/express/lib/response.js:170:12)
    at Socket.<anonymous> (/Users/amishr45/PycharmProjects/NewsVis/TrialVis/app_retrieve.js:103:7)
    at Socket.emit (events.js:198:13)
    at addChunk (_stream_readable.js:288:12)
    at readableAddChunk (_stream_readable.js:269:11)
    at Socket.Readable.push (_stream_readable.js:224:10)
    at Pipe.onStreamRead [as onread] (internal/stream_base_commons.js:94:17)

\u http\u outgoing.js:470
抛出新的ERR_HTTP_头_SENT（'set'）；
^
错误[ERR\u HTTP\u HEADERS\u SENT]：发送到客户端后无法设置头
在ServerResponse.setHeader（_http_outgoing.js:470:11）
位于ServerResponse.header（/Users/amishr45/PycharmProjects/NewsVis/TrialVis/node_modules/express/lib/response.js:771:10）
在ServerResponse.send（/Users/amishr45/PycharmProjects/NewsVis/TrialVis/node_modules/express/lib/response.js:170:12）
在插座上。（/Users/amishr45/PycharmProjects/NewsVis/TrialVis/app_retrieve.js:103:7）
在Socket.emit（events.js:198:13）
在addChunk（_stream_readable.js:288:12）
在readableAddChunk（_stream_readable.js:269:11）
在Socket.Readable.push（_stream_Readable.js:224:10）
at Pipe.onStreamRead[as onread]（内部/流基公共空间：94:17）