!node-calls-python

node-calls-python - call Python from Node.js directly in-process without spawning processes

Suitable for running your ML or deep learning models from Node directly


Motivation

Current solutions spawn a new process whenever you want to run Python code in Node.js and communicate via IPC using sockets, stdin/stdout, etc.
But creating new processes every time you want to run Python code could be a major overhead and can lead to significant performance penalties.
If the execution time of your Python code is less than creating a new process, you will see significant performance problems because your Node.js code will keep creating new processes instead of executing your Python code.
Suppose you have a few NumPy calls in Python: do you want to create a new process for that? I guess your answer is no.
In this case, running the Python code in-process is a much better solution because using the embedded Python interpreter is much faster than creating new processes and does not require any IPC to pass the data around. The data can stay in memory and requires only some conversions between Python and Node types (using the N-API and Python C API).

Installation

``
npm install node-calls-python
`

Installation FAQ

Sometimes you have to install prerequisites to make it work.

$3

#### Install Node
`
sudo apt install curl
curl -sL https://deb.nodesource.com/setup_13.x | sudo -E bash -
sudo apt install nodejs
`

#### Install Python
`
sudo apt install python3
sudo apt install python3-dev
`

#### Install Node-gyp
`
sudo apt install make
sudo apt install g++
sudo npm install -g node-gyp
`

$3

#### Install Node-gyp if missing
`
npm install --global --production windows-build-tools
npm install -g node-gyp
`

$3

`
npm install node-calls-python
`

#### If you see installation problems on Mac with ARM (E.g. using M1 Pro), try to specify 'arch' and/or 'target_arch' parameters for npm
`
npm install --arch=arm64 --target_arch=arm64 node-calls-python
`

Examples

$3

Let's say you have the following python code in test.py
`python
import numpy as np

def multiple(a, b):
return np.multiply(a, b).tolist()
`

Then to call this function directly you can do this in Node
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.import("path/to/test.py").then(async function(pymodule) {
const result = await py.call(pymodule, "multiple", [1, 2, 3, 4], [2, 3, 4, 5]);
console.log(result);
});
`

Or to call this function by using the synchronous version
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.import("path/to/test.py").then(async function(pymodule) {
const result = py.callSync(pymodule, "multiple", [1, 2, 3, 4], [2, 3, 4, 5]);
console.log(result);
});
`

$3

Let's say you have the following python code in test.py
`python
import numpy as np

class Calculator:
vector = []

def __init__(self, vector):
self.vector = vector

def multiply(self, scalar, vector):
return np.add(np.multiply(scalar, self.vector), vector).tolist()
`

Then to instance the class directly in Node
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.import("path/to/test.py").then(async function(pymodule) {
const pyobj = await py.create(pymodule, "Calculator", [1.4, 5.5, 1.2, 4.4]);
const result = await py.call(pyobj, "multiply", 2, [10.4, 50.5, 10.2, 40.4]);
});
`

Or to instance the class synchronously and directly in Node
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.import("path/to/test.py").then(async function(pymodule) {
const pyobj = py.createSync(pymodule, "Calculator", [1.4, 5.5, 1.2, 4.4]);
const result = await py.callSync(pyobj, "multiply", 2, [10.4, 50.5, 10.2, 40.4]); // you can use async version (call) as well
});
`

$3

`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.import("path/to/test.py").then(async function(pymodule) {
await py.exec(pymodule, "run_my_code(1, 2, 3)"); // exec will run any python code but the return value is not propagated
const result = await py.eval(pymodule, "run_my_code(1, 2, 3)"); // result will hold the output of run_my_code
console.log(result);
});
`

Running python code synchronously
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

const pymodule = py.importSync("path/to/test.py");
await py.execSync(pymodule, "run_my_code(1, 2, 3)"); // exec will run any python code but the return value is not propagated
const result = py.evalSync(pymodule, "run_my_code(1, 2, 3)"); // result will hold the output of run_my_code
console.log(result);
`

$3

You have to set allowReimport parameter to true when calling import/importSync.

`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

let pymodule = py.importSync("path/to/test.py");
pymodule = py.importSync("path/to/test.py", true);
`

$3

During development, you may want to update your python code running inside Node without restarting your Node process. To achieve this you can reimport your python modules.
All your python modules will be reimported where the filename of your python module matches the string parameter: `path/to/your/python/code`.
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.reimport('path/to/your/python/code');
`

Another option is to run node-calls-python in development mode. In this case, once you have updated your python code under `path/to/your/python/code` the runtime will automatically reimport the changed modules.
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.developmentMode('path/to/your/python/code');
`

$3

Javascript has no similar concept to kwargs of Python. Therefore a little hack is needed here. If you pass an object with __kwargs property set to true as a parameter to call/callSync/create/createSync the object will be mapped to kwargs.

`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

let pymodule = py.importSync("path/to/test.py");
py.callSync(pymodule, "your_function", arg1, arg2, {"name1": value1, "name2": value2, "__kwargs": true })
`

`python
def your_function(arg1, arg2, **kwargs):
print(kwargs)
`

$3

If you want to trigger a call from your Python code back to JavaScript this feature could be useful.

`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

let pymodule = py.importSync("path/to/test.py");

function jsFunction(arg1, arg2, arg3)
{
console.log(arg1, arg2, arg3);
return arg3 + 1;
}

py.callSync(pymodule, "your_function", arg1, arg2, jsFunction);
`

`python
def your_function(arg1, arg2, jsFunction):
jsResult = jsFunction(arg1 + arg2, "any string", 42);
print(jsResult);
`

You can also do this using the async API.
`javascript
py.call(pymodule, "your_function", arg1, arg2, jsFunction);
`

By default, the async Python call will wait for the execution of the JavaScript function by synchronizing the libuv thread (used by the Python call) and the main thread (used by the JavaScript function).
So the order of execution will look like this:
`
- start of py.call
- start of your_function
- start of jsFunction
- end of jsFunction
- end of your_function
- end of py.call
`

If you do not want to synchronize the execution of your JavaScript and Python code, you have to turn this off by calling setSyncJsAndPyInCallback(false) on the interpreter.
`javascript
py.setSyncJsAndPyInCallback(false);
`

In this case, one possible order of the execution could look like this (the actual order is determined by the runtime. jsFunction will run completely async).
`
- start of py.call
- start of your_function
- put jsFunction to the queue of the runtime
- end of your_function
- end of py.call
- start of jsFunction
- end of jsFunction
`

Because jsFunction runs async, it is not possible to pass the result of jsFunction back to Python. But passing arguments from Python to jsFunction is still possible.

$3

Python uses sys.executable variable when creating new processes. Because the interpreter is embedded into Node, sys.executable points to the Node executable. node-calls-python automatically overrides this setting in the multiprocessing module to point to the real Python executable. In case it does not work or you want to use a different Python executable, call setPythonExecutable(absolute-path-to-your-python-executable) before using the multiprocessing module.
`javascript
py.setPythonExecutable(absolute-path-to-your-python-executable);
`

$3

Let's say you have the following python code in logreg.py
`python
from sklearn.datasets import load_iris, load_digits
from sklearn.linear_model import LogisticRegression

class LogReg:
logreg = None

def __init__(self, dataset):
if (dataset == "iris"):
X, y = load_iris(return_X_y=True)
else:
X, y = load_digits(return_X_y=True)

self.logreg = LogisticRegression(random_state=42, solver='lbfgs', multi_class='multinomial')
self.logreg.fit(X, y)

def predict(self, X):
return self.logreg.predict_proba(X).tolist()
`

Then you can do this in Node
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.import("logreg.py")).then(async function(pymodule) { // import the python module
const logreg = await py.create(pymodule, "LogReg", "iris"); // create the instance of the classifier

const predict = await py.call(logreg, "predict", [[1.4, 5.5, 1.2, 4.4]]); // call predict
console.log(predict);
});
`

$3

You can import node-calls-python as an ES module.

`javascript
import { interpreter as py } from 'node-calls-python';

let pymodule = py.importSync(pyfile);
`

$3

If you see the following error when importing in Next.js
`Module not found: Can't resolve './build/Release/nodecallspython'`

You have to add the following code to your next.config.mjs because currently Next.js cannot bundle native node addons properly.
For more details, please see serverComponentsExternalPackages in Next.js

`
/* @type {import('next').NextConfig} /
const nextConfig = {
experimental: {
serverComponentsExternalPackages: [
'node-calls-python'
]
}
};

export default nextConfig;
`

$3

You have to add the proper import path so that python could use your installed packages from your venv.

If you have created a venv by `python -m venv your-venv` your installed python packages can be found under `your-venv/Lib/site-packages`.
So you have to use `addImportPath` before importing any module to pick up the python packages from your venv.

`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;

py.addImportPath(your-venv/Lib/site-packages)
`

$3

If you get an error like this while trying to call Python code
`ImportError: /usr/local/lib/python3.7/dist-packages/cpython-37m-arm-linux-gnueabihf.so: undefined symbol: PyExc_RuntimeError`

You can fix it by passing the name of your libpython shared library to fixlink
`javascript
const nodecallspython = require("node-calls-python");

const py = nodecallspython.interpreter;
py.fixlink('libpython3.7m.so');
`

$3

Supported data mapping

$3

`
- undefined to None
- null to None
- boolean to boolean
- number to double or long (as appropriate)
- int32 to long
- uint32 to long
- int64 to long
- string to unicode (string)
- array to list
- object to dictionary
- ArrayBuffer to bytes
- Buffer to bytes
- TypedArray to bytes
- Function to function
`

$3

`
- None to undefined
- boolean to boolean
- double to number
- long to int64
- unicode (string) to string
- list to array
- tuple to array
- set to array
- dictionary to object
- numpy.array to array (this has limited support, will convert everything to number or string)
- bytes to ArrayBuffer
- bytearray to ArrayBuffer
``