Locate the Python interpreter via the py.exe launcher

nox/virtualenv.py

@@ -62,6 +62,24 @@ def run(self, args, in_venv=True):
             path=self.bin if in_venv else None).run()
 
 
+def locate_via_py(version):
+    """Find the Python executable using the Windows launcher.
+
+    This is based on :pep:397 which details that executing
+    ``py.exe -{version}`` should execute python with the requested
+    version. We then make the python process print out its full
+    executable path which we use as the location for the version-
+    specific Python interpreter.
+    """
+    script = "import sys; print(sys.executable)"
+    py_exe = py.path.local.sysfind('py')
+    if py_exe:
+        try:
+            return py_exe.sysexec('-' + version, '-c', script).strip()
+        except py.process.cmdexec.Error:
+            return None
+
+
 class VirtualEnv(ProcessEnv):
     """Virtualenv management class."""
 
@@ -102,17 +120,15 @@ def _resolved_interpreter(self):
             return self.interpreter
 
         # If this is a standard Unix "pythonX.Y" name, it should be found
-        # in a standard location in Windows.
-        match = re.match(r'^python(?P<maj>\d)\.(?P<min>\d)$', self.interpreter)
+        # in a standard location in Windows, and if not, the py.exe launcher
+        # should be able to find it from the information in the registry.
+        match = re.match(r'^python(?P<ver>\d\.\d)$', self.interpreter)
         if match:
-            version = match.groupdict()
-            potential_paths = (
-                r'c:\python{maj}{min}\python.exe'.format(**version),
-                r'c:\python{maj}{min}-x64\python.exe'.format(**version),
-            )
-            for path in potential_paths:
-                if py.path.local(path).check():
-                    return str(path)
+            version = match.group('ver')
+            # Ask the Python launcher to find the interpreter.
+            path_from_launcher = locate_via_py(version)
+            if path_from_launcher:
+                return path_from_launcher
 
         # If we got this far, then we were unable to resolve the interpreter
         # to an actual executable; raise an exception.

tests/test_virtualenv.py

@@ -226,28 +226,53 @@ def test__resolved_interpreter_windows_full_path(make_one):
 
 
 @mock.patch.object(platform, 'system')
-@mock.patch.object(py._path.local.LocalPath, 'check')
 @mock.patch.object(py.path.local, 'sysfind')
-def test__resolved_interpreter_windows_stloc(sysfind, check, system, make_one):
-    # Establish that if we get a standard pythonX.Y path, we map it to
-    # standard locations on Windows.
+def test__resolved_interpreter_windows_pyexe(sysfind, system, make_one):
+    # Establish that if we get a standard pythonX.Y path, we look it
+    # up via the py launcher on Windows.
     venv, _ = make_one(interpreter='python3.6')
 
     # Trick the system into thinking we are on Windows.
     system.return_value = 'Windows'
 
     # Trick the system into thinking that it cannot find python3.6
-    # (it likely will on Unix).
-    sysfind.return_value = False
-
-    # Trick the system into thinking it _can_ find it in the Windows
-    # standard location.
-    check.return_value = True
+    # (it likely will on Unix). Also, when the system looks for the
+    # py launcher, give it a dummy that returns our test value when
+    # run.
+    attrs = {'sysexec.return_value': r'c:\python36\python.exe'}
+    mock_py = mock.Mock()
+    mock_py.configure_mock(**attrs)
+    sysfind.side_effect = lambda arg: mock_py if arg == 'py' else False
 
     # Okay now run the test.
     assert venv._resolved_interpreter == r'c:\python36\python.exe'
-    check.assert_called_once_with()
-    sysfind.assert_called_once_with('python3.6')
+    sysfind.assert_any_call('python3.6')
+    sysfind.assert_any_call('py')
+    system.assert_called()
+
+
+@mock.patch.object(platform, 'system')
+@mock.patch.object(py.path.local, 'sysfind')
+def test__resolved_interpreter_windows_pyexe_fails(sysfind, system, make_one):
+    # Establish that if the py launcher fails, we give the right error.
+    venv, _ = make_one(interpreter='python3.6')
+
+    # Trick the system into thinking we are on Windows.
+    system.return_value = 'Windows'
+
+    # Trick the system into thinking that it cannot find python3.6
+    # (it likely will on Unix). Also, when the system looks for the
+    # py launcher, give it a dummy that fails.
+    attrs = {'sysexec.side_effect': py.process.cmdexec.Error(1, 1, '', '', '')}
+    mock_py = mock.Mock()
+    mock_py.configure_mock(**attrs)
+    sysfind.side_effect = lambda arg: mock_py if arg == 'py' else False
+
+    # Okay now run the test.
+    with pytest.raises(RuntimeError):
+        venv._resolved_interpreter
+    sysfind.assert_any_call('python3.6')
+    sysfind.assert_any_call('py')
     system.assert_called()