Dowsing Rod

Dowse for Python 2.7 installations on your system and make them available to node-gyp subprocesses.

Why?

node-gyp only works with Python 2, and this is unlikely to change in the near future.

Furthermore, while node-gyp allows you to override the Python executable it discovers itself by specifying a PYTHON environment variable or setting the python npm config option, certain node-gyp builds will only work correctly if the python 2 executable is on the ${PATH} with the exact name "python", because the Makefiles they generate execute generated files with a shebang line of #!/usr/bin/env python.

Fixing this upstream would be tricky and require changes to trickle through a number of dependent projects. Hence @atom/dowsing-rod, which exists to:

• Locate a compatible Python 2 executable (a) from an existing $PYTHON variable (b) as any python, python2, or python2.7 executable on your $PATH or (c) specified directly as a .python2-bin file.
• Set the $PYTHON environment variable to the discovered binary's path. Prepend a directory containing a wrapper script to your $PATH so that shebang scripts will execute your chosen Python binary properly.

Install

$ npm install @atom/dowsing-rod

You'll also likely want to add .python2-bin to your .gitignore:

$ echo '.python2-bin' >> .gitignore

Use

From your package.json

To fix your node-gyp rebuild command, add the following to your package.json:

{
  "scripts": {
    "install": "with-python2-env node-gyp rebuild"
  }
}

From code

If you're launching node-gyp as a subprocess yourself (like we do within the build scripts, for example), you can modify your process' environment directly:

const path = require('path')
const dowsingRod = require('@atom/dowsing-rod')

const options = {
  // npm binary used to read config settings
  npmBin: 'npm',

  // file to store located Python binary
  pythonBinFile: path.join(process.cwd(), '.python2-bin')
}

// Options are optional with the defaults shown.
const promise = dowsingRod.setPythonEnv(options, process.env)

// Any exec or spawn calls after the returned promise resolves will have the correct Python environments.
// Or, if you wish to do something with the path yourself:

dowsingRod.getPythonBin(options)
  .then(pythonBin => console.log(`Python binary: ${pythonBin}`))

// Synchronous variants are also exported:
dowsingRod.setPythonEnvSync(options, process.env)
const pythonBin = dowsingRod.getPythonBinSync(options)