Returning data from Python to node.js

Question

I've been following along with a couple examples I found to call a Python script from Node. I'm able to execute the script, but I can't return data from Python.

test.js

var sys   = require('sys'),
    spawn = require('child_process').spawn,
    dummy  = spawn('python', ['test.py']);

dummy.stdout.on('data', function (data) {
sys.print("testing...\n");
sys.print(data);
});

test.py

import time
def dummy() :
    out = '';
    for i in range(0,10) :
        out += str(i + 1) + "\n"
        time.sleep(0.1)
    print out
    return out
if __name__ =='__main__' :
    dummy = dummy()

Could someone provide an example of how to return the results from test.py to test.js?

Thanks.

Note: test.py edited for proper indent.

I think my question comes down to this: How do I either return a value from python's main() (if name == 'main'), or how do I call a specific python method (dummy()) from Node? — RobertFrenette
– RobertFrenette, Commented Dec 30, 2013 at 0:39

RobertFrenette · Accepted Answer · 2013-12-31 19:12:31Z

4

I figured out how to do this, by printing the data in Python, and "listening" for the print in Node using stdout.on().

test.js

var sys   = require('sys'),
    spawn = require('child_process').spawn,
    dummy  = spawn('python', ['test.py']);

dummy.stdout.on('data', function(data) {
    sys.print(data.toString());
});

test.py

import time
def dummy() :
    out = '';
    for i in range(0,10) :
        out += str(i + 1) + ", "
        time.sleep(0.1)
    print out
if __name__ =='__main__' :
    dummy = dummy()

answered Dec 31, 2013 at 19:12

RobertFrenette

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Comments

Hariharnath Paduchuru · Accepted Answer · 2017-12-12 05:29:20Z

3

As the Sys is deprecated, we can use util. Please find the modified code using util

test.js

var sys   = require('util'),
    spawn = require('child_process').spawn,
    dummy  = spawn('python', ['test.py']);

dummy.stdout.on('data', function(data) {
    console.log(data.toString());
});

test.py

import time
def dummy() :
    out = '';
    for i in range(0,10) :
        out += str(i + 1) + ", "
        time.sleep(0.1)
    print out
if __name__ =='__main__' :
    dummy = dummy()

answered Dec 12, 2017 at 5:29

Hariharnath Paduchuru

1376 bronze badges

Comments

damphat · Accepted Answer · 2013-12-29 18:57:52Z

1

I think your test.py has bug. You should check again for the indents, then run python test.py in your terminal to see if it print 1 to 10 or not:

import time
def dummy() :
  out = '';
  for i in range(0,10) :
    out += str(i + 1) + "\n"
    time.sleep(0.1)
  print out
  return out

if __name__ =='__main__' :
    dummy = dummy()

You should redirect both stdout, stderr to parent process, that help you quickly identify the bug:

var spawn = require('child_process').spawn,
var dummy  = spawn('python', ['test.py']);

dummy.stdout.pipe(process.stdout);
dummy.stderr.pipe(process.stderr);

answered Dec 29, 2013 at 18:57

damphat

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5 Comments

RobertFrenette Over a year ago

Thanks for the reply. I ran python test.py and it did print 1 - 10.

damphat Over a year ago

Did you get any message when run node test.js?

RobertFrenette Over a year ago

No, there are no errors or messages. Both test.js and test.py seem to complete normally.

damphat Over a year ago

as I see, the python code in your question has wrong indent. And if test.js is not print result, I think this line of code dummy.stderr.pipe(process.stderr); would print the error.

RobertFrenette Over a year ago

Oh, I see what happened. You are right, the indents in my post aren't correct. I went back and looked at the code in my editor, and it runs fine. I didn't indent properly after I copied/pasted for my original post. So, when I run the code as I posted above, I do get an error, as you say. When I run it as it is in my editor, with proper indents, I see 1 - 10 printed out. My apologies for the improper indents in my post.

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