Prevent conflicting AMBuild installations.

[dvander]

This detects older AMBuild installs that may have been installed with
distutils, and forces their removal.

[WildCard65]

Optional A: Use a sentinel object

sentinel = object() # Plain object for minimal memory usage.
if getattr(ambuild2.util, 'INSTALLED_BY_PIP_OR_SETUPTOOLS', sentinel) is not sentinel:

Option B: Catch "AttributeError"

try:
  obj = getattr(ambuild2.util, 'INSTALLED_BY_PIP_OR_SETUPTOOLS')
except AttributeError:
  pass
else:
  sys.exit(2)

Option A is less weird looking, Option B is more "pythonic"

[WildCard65]

Sanity check:
Python 2.7+ confirms "object()" results in new objects, not a cached object.

[dvander]

I'm trying to keep this simple - Python is... not my first choice anymore, and maintaining AMBuild is hard. I have to remember a lot of quirks. It should have been written in Go, but that wasn't around when AMBuild was needed. Anyway, "True" is a lot more clear than "object()", especially given the boolean phrasing of the variable name.

[WildCard65]

Nitpicks:

• Use a tuple, candidates is implied to be immutable.
• A generator expression would also suffice.

[dvander]

Similarly, I try to be consistent in using lists over tuples. Tuples are quirky. It's not clear why most code would care about immutability.

[WildCard65]

Other than the changes I suggested (Nitpick review not included), I'm fine with doing this @dvander.

[dvander]

Thanks for taking a look, hopefully no one runs into any problems.

[WildCard65]

@dvander It seems you're using multiprocessing wrong according to my local Python install:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Program Files\Python\3.8\lib\multiprocessing\spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "C:\Program Files\Python\3.8\lib\multiprocessing\spawn.py", line 125, in _main
    prepare(preparation_data)
  File "C:\Program Files\Python\3.8\lib\multiprocessing\spawn.py", line 236, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Program Files\Python\3.8\lib\multiprocessing\spawn.py", line 287, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
  File "C:\Program Files\Python\3.8\lib\runpy.py", line 265, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "C:\Program Files\Python\3.8\lib\runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "C:\Program Files\Python\3.8\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "L:\GIT\ambuild\setup.py", line 25, in <module>
    proc.start()
  File "C:\Program Files\Python\3.8\lib\multiprocessing\process.py", line 121, in start
    self._popen = self._Popen(self)
  File "C:\Program Files\Python\3.8\lib\multiprocessing\context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "C:\Program Files\Python\3.8\lib\multiprocessing\context.py", line 327, in _Popen
    return Popen(process_obj)
  File "C:\Program Files\Python\3.8\lib\multiprocessing\popen_spawn_win32.py", line 45, in __init__
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "C:\Program Files\Python\3.8\lib\multiprocessing\spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "C:\Program Files\Python\3.8\lib\multiprocessing\spawn.py", line 134, in _check_not_importing_main
    raise RuntimeError('''
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

[WildCard65]

According to the docs, you need to insert a 'freeze_support()' call before creating the Process object.