#890 Add support for the Python launcher on Windows

[poke]

This is a pull request for the issue #890: “Provide fallback to Python launcher on Windows when python.exe cannot be found”.

The changes here attempt to have very little impact. The new check is just inserted between the checkPython call (looking for the 'python' executable) and the guessPython call on Windows. Since the Python launcher will give an explicit Python 2 executable, it is more explicit than if we found a match using guessPython, so I gave the launcher solution more priority.

As explained in the commit message, we cannot make a call to py.exe -2 since that wouldn’t work with execFile, so I had to make an actual call to the Python launcher to get the full path of the Python executable. But if the command succeeds, then that reliably gives us a valid Python 2 executable.

If there is no Python 2 version registered with the launcher, the launcher will fail when attempting to call it with -2. When there is no Python launcher, execFile will fail. In both cases, the old next step guessPython will run.

When looking for a Python executable on Windows, before falling back to guessing the default location or failing completely, attempt to use the Python launcher to figure out the location of the Python executable.

The Python launcher is being distributed by default with Python distributions on Windows, and is placed in the %WINDIR% folder (which is in the PATH). This allows us to locate a Python installation even if it was installed without putting the python.exe executable itself into the PATH.

Because the Python launcher supports all versions of Python, we have to explicitly request a Python 2 version. This is done by supplying "-2" as the first command line argument. Since "py.exe -2" would be an invalid executable for "execFile", we have to use the launcher to figure out where the actual "python.exe" executable is located.

[bnoordhuis]

Can you wrap this line at 80 columns?

[bnoordhuis]

Mostly LGTM, thanks. Can you verify that npm test still passes and maybe add one or two tests to test/test-find-python.js if possible?

[poke]

Updated my pull request to wrap at 80 characters; for what it’s worth, the checkPythonVersion a few lines below of my code is happily breaking the 80 characters limit too – do you want me to adjust that too?

As for the test, with my changes, npm test does pass for me. It did not before since the find python executable failed on my machine (for the exact reasons why I am submitting this pull request). As such I’m also not sure how I could add additional tests for finding the python executable since it highly depends on the environment the test is run on.

[bnoordhuis]

LGTM and, seeing how there are no comments from other maintainers, I'll go ahead and land this. Thanks Patrick, merged in 3bcb172.

[poke]

Glad to see this land, thanks a lot! :)