Could not detect the Python executable based on `npm config ls`

I am on Windows, and I have Python 3 in the command line for general Python programming.

$ python --version
Python 3.5.1 :: Continuum Analytics, Inc.

To enable node-gyp, I use windows-build-tools to install Python 2 and configure npm as desired:

$ npm config ls
scope = ""
user-agent = "npm/4.3.0 node/v6.3.1 win32 x64"

msvs_version = "2015"
python = "C:\\Users\\username\\.windows-build-tools\\python27\\python.exe"

prefix = "C:\\Users\\username\\AppData\\Roaming\\npm"

I still get the following error while running electron-rebuild because electron-rebuild could not find the correct Python 2 based on npm config ls:

$ npm install zerorpc # the package I could not bind to electron
$ npm install electron electron-rebuild

$ ./node_modules/.bin/electron-rebuild
× Rebuild Failed
An unhandled error occurred inside electron-rebuild
gyp info it worked if it ends with ok
gyp info using node-gyp@3.5.0
gyp info using node@6.3.1 | win32 | x64
gyp ERR! configure error
gyp ERR! stack Error: Can't find Python executable "C:\Anaconda3\python.EXE", you can set the PYTHON env variable.
...

Expected result: no errors, and the package could detect the Python 2 executable properly.

[abhideshmukh]

It is not compatible with python 3.x.x . Try to install ^2.7.x (the lest one).

[autotel]

For people who have the latest version of python, the error message doesn’t help much. It should tell that there is no python, or its version is not compatible.