Python and Node js communication

Question

I have been trying to communicate between Node JS and Python following this tutorial: http://www.sohamkamani.com/blog/2015/08/21/python-nodejs-comm/

The python file reads a javascript array and prints the sum using the numpy module. Here is the code for both the python and node js file.

Python code:

 import sys, json, numpy as np
 #Read data from stdin
 def read_in():
 lines = sys.stdin.readlines()
 return json.loads(lines[0])
 def main():
 #get our data as an array from read_in()
 lines = read_in()
 #create a numpy array
 np_lines = np.array(lines)
 #use numpys sum method to find sum of all elements in the array
 lines_sum = np.sum(np_lines)
 #return the sum to the output stream
 print lines_sum
 #start process
 if __name__ == '__main__':
 main()

Node js code:

 var spawn = require('child_process').spawn,
 py =spawn('python', ['compute_input.py']),
 data = [1,2,3,4,5,6], 
 dataString='';
 py.stdout.on('data', function(data) {
 dataString += data.toString();
 });
 py.stdout.on('end', function() {
 console.log('Sum = ', dataString);
 });
 py.stdin.write(JSON.stringify(data));
 py.stdin.end();

Error message:

 Error: write EPIPE
 at exports._errnoException (util.js:1018:11)
 at Socket._writeGeneric (net.js:711:26)
 at Socket._write (net.js:730:8)
 at doWrite (_stream_writable.js:331:12)
 at writeOrBuffer (_stream_writable.js:317:5)
 at Socket.Writable.write (_stream_writable.js:243:11)
 at Socket.write (net.js:657:40) 
 at Module._compile (module.js:570:32)
 at Object.Module._extensions..js (module.js:579:10)

• MB-F: (1) This is not valid Python code. (2) How do you run the scripts?
• Akshay: (1) Can you clarify what is not valid? (2) I am running the scripts with the node js command prompt.
• MB-F: The function definitions like **def read_in():** should not contain these stars. If you run the Python script separately the interpreter would tell you. I guess node swallows the error message from the child process.
• Akshay: I understand that python functions do not use **. I wanted it to be clear where the functions were. Please see revised format.
• MB-F: It's clearer without them :) Have you tried running the Python script manually? There may be less obvious errors.

Answer 1

The Python script is throwing an error. The python command invoked python3 instead of python 2.7, for which the script was written and the script errored out. EPIPE error is thrown because the python script ended prematurely and node tries to read-from/write-to to an already closed pipe.