python-shell

A simple way to run Python scripts from Node.js with basic but efficient inter-process communication and better error handling.

Features

• Reliably spawn Python scripts in a child process
• Built-in text, JSON and binary modes
• Custom parsers and formatters
• Simple and efficient data transfers through stdin and stdout streams
• Extended stack traces when an error is thrown

Installation

npm install python-shell

To run the tests:

npm test

Documentation

Running a Python script:

var PythonShell = require('python-shell');
 
PythonShell.run('my_script.py', function (err) {
  if (err) throw err;
  console.log('finished');
});

If the script writes to stderr or exits with a non-zero code, an error will be thrown.

Running a Python script with arguments and options:

var PythonShell = require('python-shell');
 
var options = {
  mode: 'text',
  pythonPath: 'path/to/python',
  pythonOptions: ['-u'],
  scriptPath: 'path/to/my/scripts',
  args: ['value1', 'value2', 'value3']
};
 
PythonShell.run('my_script.py', options, function (err, results) {
  if (err) throw err;
  // results is an array consisting of messages collected during execution 
  console.log('results: %j', results);
});

Exchanging data between Node and Python:

var PythonShell = require('python-shell');
var pyshell = new PythonShell('my_script.py');
 
// sends a message to the Python script via stdin 
pyshell.send('hello');
 
pyshell.on('message', function (message) {
  // received a message sent from the Python script (a simple "print" statement) 
  console.log(message);
});
 
// end the input stream and allow the process to exit 
pyshell.end(function (err) {
  if (err) throw err;
  console.log('finished');
});

Use .send(message) to send a message to the Python script. Attach the message event to listen to messages emitted from the Python script.

Use options.mode to quickly setup how data is sent and received between your Node and Python applications.

• use text mode for exchanging lines of text
• use json mode for exchanging JSON fragments
• use binary mode for anything else (data is sent and received as-is)

For more details and examples including Python source code, take a look at the tests.

Error Handling and extended stack traces

An error will be thrown if the process exits with a non-zero exit code or if data has been written to stderr. Additionally, if "stderr" contains a formatted Python traceback, the error is augmented with Python exception details including a concatenated stack trace.

Sample error with traceback (from test/python/error.py):

Traceback (most recent call last):
  File "test/python/error.py", line 6, in <module>
    divide_by_zero()
  File "test/python/error.py", line 4, in divide_by_zero
    print 1/0
ZeroDivisionError: integer division or modulo by zero

would result into the following error:

{ [Error: ZeroDivisionError: integer division or modulo by zero]
  traceback: 'Traceback (most recent call last):\n  File "test/python/error.py", line 6, in <module>\n    divide_by_zero()\n  File "test/python/error.py", line 4, in divide_by_zero\n    print 1/0\nZeroDivisionError: integer division or modulo by zero\n',
  executable: 'python',
  options: null,
  script: 'test/python/error.py',
  args: null,
  exitCode: 1 }

and err.stack would look like this:

Error: ZeroDivisionError: integer division or modulo by zero
    at PythonShell.parseError (python-shell/index.js:131:17)
    at ChildProcess.<anonymous> (python-shell/index.js:67:28)
    at ChildProcess.EventEmitter.emit (events.js:98:17)
    at Process.ChildProcess._handle.onexit (child_process.js:797:12)
    ----- Python Traceback -----
    File "test/python/error.py", line 6, in <module>
      divide_by_zero()
    File "test/python/error.py", line 4, in divide_by_zero
      print 1/0

API Reference

PythonShell(script, options) constructor

Creates an instance of PythonShell and starts the Python process

• script: the path of the script to execute
• options: the execution options, consisting of:
  • mode: Configures how data is exchanged when data flows through stdin and stdout. The possible values are:
    • text: each line of data (ending with "\n") is emitted as a message (default)
    • json: each line of data (ending with "\n") is parsed as JSON and emitted as a message
    • binary: data is streamed as-is through stdout and stdin
  • formatter: each message to send is transformed using this method, then appended with "\n"
  • parser: each line of data (ending with "\n") is parsed with this function and its result is emitted as a message
  • encoding: the text encoding to apply on the child process streams (default: "utf8")
  • pythonPath: The path where to locate the "python" executable. Default: "python"
  • pythonOptions: Array of option switches to pass to "python"
  • scriptPath: The default path where to look for scripts. Default is the current working directory.
  • args: Array of arguments to pass to the script

Other options are forwarded to child_process.spawn.

PythonShell instances have the following properties:

• script: the path of the script to execute
• command: the full command arguments passed to the Python executable
• stdin: the Python stdin stream, used to send data to the child process
• stdout: the Python stdout stream, used for receiving data from the child process
• stderr: the Python stderr stream, used for communicating errors
• childProcess: the process instance created via child_process.spawn
• terminated: boolean indicating whether the process has exited
• exitCode: the process exit code, available after the process has ended

Example:

// create a new instance 
var shell = new PythonShell('script.py', options);

#defaultOptions

Configures default options for all new instances of PythonShell.

Example:

// setup a default "scriptPath" 
PythonShell.defaultOptions = { scriptPath: '../scripts' };

#run(script, options, callback)

Runs the Python script and invokes callback with the results. The callback contains the execution error (if any) as well as an array of messages emitted from the Python script.

This method is also returning the PythonShell instance.

Example:

// run a simple script 
PythonShell.run('script.py', function (err, results) {
  // script finished 
});

.send(message)

Sends a message to the Python script via stdin. The data is formatted according to the selected mode (text or JSON), or through a custom function when formatter is specified.

Example:

// send a message in text mode 
var shell = new PythonShell('script.py', { mode: 'text '});
shell.send('hello world!');
 
// send a message in JSON mode 
var shell = new PythonShell('script.py', { mode: 'json '});
shell.send({ command: "do_stuff", args: [1, 2, 3] });

.receive(data)

Parses incoming data from the Python script written via stdout and emits message events. This method is called automatically as data is being received from stdout.

.end(callback)

Closes the stdin stream, allowing the Python script to finish and exit. The optional callback is invoked when the process is terminated.

event: message

Fires when a chunk of data is parsed from the stdout stream via the receive method. If a parser method is specified, the result of this function will be the message value. This event is not emitted in binary mode.

Example:

// receive a message in text mode 
var shell = new PythonShell('script.py', { mode: 'text '});
shell.on('message', function (message) {
  // handle message (a line of text from stdout) 
});
 
// receive a message in JSON mode 
var shell = new PythonShell('script.py', { mode: 'json '});
shell.on('message', function (message) {
  // handle message (a line of text from stdout, parsed as JSON) 
});

event: close

Fires when the process has been terminated, with an error or not.

event: error

Fires when the process terminates with a non-zero exit code, or if data is written to the stderr stream.

License

The MIT License (MIT)

Copyright (c) 2014 Nicolas Mercier

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.