python-bridge

python-bridge

Most robust and simple Python bridge. Features, and comparisons to other Python bridges below.

npm install python-bridge

'use strict';

let assert = require('assert');
let pythonBridge = require('python-bridge');

let python = pythonBridge();

python.ex`import math`;
python`math.sqrt(9)`.then(x => assert.equal(x, 3));

let list = [3, 4, 2, 1];
python`sorted(${list})`.then(x => assert.deepEqual(x, list.sort()));

python.end();

API

var python = pythonBridge(options)

Spawns a Python interpreter, exposing a bridge to the running processing. Configurable via options.

• options.python - Python interpreter, defaults to python

Also inherits the following from child_process.spawn([options]).

• options.cwd - String Current working directory of the child process
• options.env - Object Environment key-value pairs
• options.stdio - Array Child's stdio configuration. Defaults to ['pipe', process.stdout, process.stderr]
• options.uid - Number Sets the user identity of the process.
• options.gid - Number Sets the group identity of the process.

var python = pythonBridge({
    python: 'python3',
    env: {PYTHONPATH: '/foo/bar'}
});

python`expression(args...)`.then(...)

Evaluates Python code, returning the value back to Node.

// Interpolates arguments using JSON serialization.
python`sorted(${[6, 4, 1, 3]})`.then(x => assert.deepEqual(x, [1, 3, 4, 6]));

// Passing key-value arguments
let obj = {hello: 'world', foo: 'bar'};
python`dict(baz=123, **${obj})`.then(x => {
    assert.deepEqual(x, {baz: 123, hello: 'world', foo: 'bar'});
});

python.ex`statement`.then(...)

Execute Python statements.

let a = 123, b = 321;
python.ex`
    def hello(a, b):
        return a + b
`;
python`hello(${a}, ${b})`.then(x => assert.equal(x, a + b));

python.lock(...).then(...)

Locks access to the Python interpreter so code can be executed atomically. If possible, it's recommend to define a function in Python to handle atomicity.

python.lock(python => {
    python.ex`hello = 123`;
    return python`hello + 321'`;
}).then(x => assert.equal(x, 444));

// Recommended to define function in Python
python.ex`
    def atomic():
        hello = 123
        return hello + 321
`;
python`atomic()`.then(x => assert.equal(x, 444));

python.stdin, python.stdout, python.stderr

Pipes going into the Python process, separate from execution & evaluation. This can be used to stream data between processes, without buffering.

let Promise = require('bluebird');
let fs = Promise.promisifyAll(require('fs'));

let fileWriter = fs.createWriteStream('output.txt');

python.stdout.pipe(fileWriter);

// listen on Python process's stdout
python.ex`
    import sys
    for line in sys.stdin:
        sys.stdout.write(line)
        sys.stdout.flush()
`.then(function () {
    fileWriter.end();
    fs.readFileAsync('output.txt', {encoding: 'utf8'}).then(x => assert.equal(x, 'hello\nworld\n'));
});

// write to Python process's stdin
python.stdin.write('hello\n');
setTimeout(() => {
    python.stdin.write('world\n');
    python.stdin.end();
}, 10);

python.end()

Stops accepting new Python commands, and waits for queue to finish then gracefully closes the Python process.

python.disconnect()

Alias to python.end()

python.kill([signal])

Send signal to Python process, same as child_process child.kill.

let Promise = require('bluebird');

python.ex`
    from time import sleep
    sleep(9000)
`.timeout(100).then(x => {
    assert.ok(false);
}).catch(Promise.TimeoutError, (exit_code) => {
    console.error('Python process taking too long, restarted.');
    python.kill('SIGKILL');
    python = pythonBridge();
});

Handling Exceptions

We can use Bluebird's promise.catch(...) catch handler in combination with Python's typed Exceptions to make exception handling easy.

python.Exception

Catch any raised Python exception.

python.ex`
    hello = 123
    print(hello + world)
    world = 321
`.catch(python.Exception, () => console.log('Woops!  `world` was used before it was defined.'));

python.isException(name)

Catch a Python exception matching the passed name.

function pyDivide(numerator, denominator) {
    return python`${numerator} / ${denominator}`
        .catch(python.isException('ZeroDivisionError'), () => Promise.resolve(Infinity));
}
pyDivide(1, 0).then(x => {
    assert.equal(x, Infinity);
    assert.equal(1 / 0, Infinity);
});

pythonBridge.PythonException

Alias to python.Exception, this is useful if you want to import the function to at the root of the module.

pythonBridge.isPythonException

Alias to python.isException, this is useful if you want to import the function to at the root of the module.

Features

• Does not affect Python's stdin, stdout, or stderr pipes.
• Exception stack traces forwarded to Node for easy debugging.
• Python 2 & 3 support, end-to-end tested.
• Command queueing, with promises.
• Long running Python sessions.
• ES6 template tags for easy interpolation & multiline code.

Comparisons

After evaluating of the existing landscape of Python bridges, the following issues are why python-bridge was built.

• python-shell — No promises for queued requests; broken evaluation parser; conflates evaluation and stdout; complex configuration.
• python — Broken evaluation parsing; no exception handling; conflates evaluation, stdout, and stderr.
• node-python — Complects execution protocol with incomplete Python embedded DSL.
• python-runner — No long running sessions; child_process.spawn wrapper with unintuitive API; no serialization.
• python.js — Embeds specific version of CPython; requires compiler and CPython dev packages; incomplete Python embedded DSL.
• cpython — Complects execution protocol with incomplete Python embedded DSL.
• eval.py — Can only evaluate single line expressions.
• py.js — For setting up virtualenvs only.

License

MIT