tox.ini
[testenv:py36-integration]
basepython = "/this/should/be/a/link/to/python3.6.5"
commands =
;    py.test tests/test_integration.py {posargs:-vvs}
    py.test tests/test_integration.py {posargs}

def test_interpreter_resolution_without_pex_python_path():
  # tox -e py36-integration -- -s -k test_interpreter_resolution_without_pex_python_path

  # We want a test where:
  #   We build a pex with some interpreter-constraints set
  #   At construction time, we don't want either PEX_PYTHON or PEX_PYTHON_PATH set
  with temporary_dir() as td:
    pex_out_path = os.path.join(td, 'pex.pex')
    res = run_pex_command(['--disable-cache',
      '--interpreter-constraint===%s' % PY36,
      '-o', pex_out_path],
      # env=make_env(PATH="/usr/local/bin/python3.6") # 3.6.6
    )
    res.assert_success()
  #   We want to try to run that pex with no environment variables set
    stdin_payload = b'import sys; print(sys.executable); sys.exit(0)'

    # We want this to fail because it won't find python3.6.6 in the PATH, and it's looking for it.
    stdout, rc = run_simple_pex(pex_out_path, stdin=stdin_payload)
    assert rc == 1

--interpreter-constraints