Fix: try python3 before python in install --system (pypa#5296)

Fix a regression from commit dbea3f5 where `pip install --system` tries
to install using the `python` executable first which, on some systems
may point to Python 2.  Instead try `python3` first.

news/5296.bugfix.rst

@@ -0,0 +1 @@
+Fix an issue when using ``pipenv install --system`` on systems that having the ``python`` executable pointing to Python 2 and a Python 3 executable being ``python3``.

pipenv/utils/shell.py

@@ -445,7 +445,7 @@ def project_python(project, system=False):
  if not system:
  python = project._which("python")
  else:
- interpreters = [system_which(p) for p in ("python", "python3")]
+ interpreters = [system_which(p) for p in ("python3", "python")]
  interpreters = [i for i in interpreters if i] # filter out not found interpreters
  python = interpreters[0] if interpreters else None
  if not python:

tests/unit/test_utils.py

@@ -458,3 +458,17 @@ def test_invalid_prepare_pip_source_args(self):
  sources = [{}]
  with pytest.raises(PipenvUsageError):
  indexes.prepare_pip_source_args(sources, pip_args=None)
+
+ @pytest.mark.utils
+ def test_project_python_tries_python3_before_python_if_system_is_true(self):
+ def mock_shutil_which(command, path=None):
+ if command != "python3":
+ return f"/usr/bin/{command}"
+ return "/usr/local/bin/python3"
+
+ with mock.patch("pipenv.utils.shell.shutil.which", wraps=mock_shutil_which):
+ # Setting project to None as system=True doesn't use it
+ project = None
+ python = shell.project_python(project, system=True)
+
+ assert python == "/usr/local/bin/python3"