Node.js child process to Python process

Question

I must send text from a node.js child process to a python process. My dummy node client looks like

var resolve = require('path').resolve;
var spawn = require('child_process').spawn;

data = "lorem ipsum"

var child = spawn('master.py', []);

var res = '';
child.stdout.on('data', function (_data) {
    try {
        var data = Buffer.from(_data, 'utf-8').toString();
        res += data;
    } catch (error) {
        console.error(error);
    }
});
child.stdout.on('exit', function (_) {
    console.log("EXIT:", res);
});
child.stdout.on('end', function (_) {
    console.log("END:", res);
});
child.on('error', function (error) {
    console.error(error);
});

child.stdout.pipe(process.stdout);

child.stdin.setEncoding('utf-8');
child.stdin.write(data + '\r\n');

while the Python process master.py is

#!/usr/bin/env python

import sys
import codecs

if sys.version_info[0] >= 3:
    ifp = codecs.getreader('utf8')(sys.stdin.buffer)
else:
    ifp = codecs.getreader('utf8')(sys.stdin)

if sys.version_info[0] >= 3:
    ofp = codecs.getwriter('utf8')(sys.stdout.buffer)
else:
    ofp = codecs.getwriter('utf8')(sys.stdout)

for line in ifp:
    tline = "<<<<<" + line + ">>>>>"
    ofp.write(tline)

# close files
ifp.close()
ofp.close()

I must use a utf-8 encoded input reader so I'm using a sys.stdin, but it seems that when node.js writes to child process stdin using child.stdin.write(data + '\r\n');, this will not be read by sys.stdin in for line in ifp:

ottomeister · Accepted Answer · 2019-02-16 07:33:41Z

2

You'll need to call child.stdin.end() in the Node program after the final call to child.stdin.write(). Until end() is called, the child.stdin writable stream will hold the written data in a buffer, so the Python program won't see it. See the Buffering discussion in https://nodejs.org/docs/latest-v8.x/api/stream.html#stream_buffering for details.

(If you write lots of data into stdin then the write buffer will eventually fill to a point where the accumulated data will be flushed out automatically to the Python program. The buffer will then begin again to collect data. An end() call is needed to make sure that the final portion of the written data is flushed out. It also has the effect of indicating to the child process that no more data will be sent on this stream.)

answered Feb 16, 2019 at 7:33

ottomeister

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2 Comments

loretoparisi Over a year ago

Thank you that's correct. Supposed that I would like to keep the stdin open, so that I could do multiple child.stdin.write, how can I force to flush the buffer so that the python will send back the accumulated data?

loretoparisi Over a year ago

I have created a specific question for this: stackoverflow.com/questions/54725072/…

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