Unexpected python executable can be used by rez-pip

[SuperValou]

In some cases, rez-pip can unexpectedly use a python executable found on your machine instead of a rezified python package.

Environment

• Windows 10.0
• rez-pip 0.2.0
• rez-pip being installed in its own virtual environment, not part of a rez installation

To Reproduce

• Create an empty file named python3.exe in some arbitrary folder, e.g. "C:\foo\python3.exe"
• Rezify a python interpreter through this utility
• Install rez-pip into its own virtualenv
  • open a terminal
  • run rez env python -- python -m venv C:\rez-pip to generate a virtualenv that will host the rez-pip package
  • run cd C:\rez-pip\Scripts then .\activate to enter the virtualenv
  • run python -m pip install rez-pip to install rez-pip in the virtualenv
• Through the Windows Configuration panel, add "C:\foo" at the end of your PATH environment variable
• open a terminal
• run cd C:\rez-pip\Scripts then .\activate to enter the virtualenv of rez-pip
• run .\rez-pip2.exe future
• see the error:

> .\rez-pip2.exe future
INFO     Installing requested packages for Python 3.7.7
Traceback (most recent call last):
  File "\\ ... \packages\python\3.9.10\platform-windows\arch-AMD64\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "\\ ... \packages\python\3.9.10\platform-windows\arch-AMD64\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\rez-pip\Scripts\rez-pip2.exe\__main__.py", line 7, in <module>
  File "C:\rez-pip\lib\site-packages\rez_pip\cli.py", line 318, in run
    _run(args, pipArgs, pipWorkArea)
  File "C:\rez-pip\lib\site-packages\rez_pip\cli.py", line 176, in _run
    packages = rez_pip.pip.getPackages(
  File "C:\rez-pip\lib\site-packages\rez_pip\pip.py", line 106, in getPackages
    process = subprocess.Popen(
  File "\\ ... \packages\python\3.9.10\platform-windows\arch-AMD64\lib\subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "\\ ... \packages\python\3.9.10\platform-windows\arch-AMD64\lib\subprocess.py", line 1420, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
OSError: [WinError 193] %1 not a valid Win32 application

Expected behavior
Program installs future as a rez package.

Actual behavior
The exception above occurs.

Investigation
The error is due to this line checking for a "python3.exe" in PATH before checking for "python.exe". The empty python3.exe we created gets found before the python.exe of our rezified python.

Possible suggestions

• If the rezified python package provides python in its tools section, directly use "python" instead of attempting "python3.7", "python3" then "python"
• check that the result of which is a under the root of a rezified python package

Note
IMO being able to use an arbitrary python executable instead of the one provided by a rezified python package could be interesting, but it should be an explicit behaviour requested by the user; the default behavior of rez-pip should be to only use a rezified python package.

[SuperValou]

Attempting to pass parent_environ={} to which (here) doesn't work because ultimately the ResolvedContext object creates a RexExecutor that appends PATH anyway. See here to check the call stack at the point where PATH gets added

[SuperValou]

Maybe we can replicate the behaviour of find_python_in_context by setting ResolvedContext.append_sys_path = False (like here) right before this line

[JeanChristopheMorinPerso]

@SuperValou I created a PR, #47, could you try it to see if it works as you expect please?

[SuperValou]

@JeanChristopheMorinPerso Yes it does! Thanks 😃