verify Python is runnable

internal/environment/python.go

@@ -55,6 +55,12 @@ func NewPythonInspector(projectDir util.Path, pythonPath util.Path, log logging.
 	}
 }
 
+func (i *defaultPythonInspector) validatePythonExecutable(pythonExecutable string) error {
+	args := []string{"--version"}
+	_, err := i.executor.runPythonCommand(pythonExecutable, args)
+	return err
+}
+
 func (i *defaultPythonInspector) getPythonExecutable(exec util.PathLooker) (string, error) {
 	if i.pythonPath.Path() != "" {
 		// User-provided python executable
@@ -71,10 +77,23 @@ func (i *defaultPythonInspector) getPythonExecutable(exec util.PathLooker) (stri
 	} else {
 		// Use whatever is on PATH
 		path, err := exec.LookPath("python3")
+		if err == nil {
+			// Ensure the Python is actually runnable. This is especially
+			// needed on Windows, where `python3` is (by default)
+			// an app execution alias. Also, installing Python from
+			// python.org does not disable the built-in app execution aliases.
+			err = i.validatePythonExecutable(path)
+		}
 		if err != nil {
-			return exec.LookPath("python")
+			path, err = exec.LookPath("python")
+			if err == nil {
+				err = i.validatePythonExecutable(path)
+			}
+		}
+		if err != nil {
+			return "", err
 		}
-		return path, err
+		return path, nil
 	}
 }
 

internal/environment/python_test.go

@@ -108,16 +108,83 @@ func (s *PythonSuite) TestGetPythonVersionFromRealDefaultPython() {
 	s.True(strings.HasPrefix(version, "3."))
 }
 
+type mockPythonExecutor struct {
+	mock.Mock
+}
+
+var _ pythonExecutor = &mockPythonExecutor{}
+
+func (m *mockPythonExecutor) runPythonCommand(pythonExecutable string, args []string) ([]byte, error) {
+	mockArgs := m.Called(pythonExecutable, args)
+	out := mockArgs.Get(0)
+	if out != nil {
+		return out.([]byte), mockArgs.Error(1)
+	} else {
+		return nil, mockArgs.Error(1)
+	}
+}
+
 func (s *PythonSuite) TestGetPythonExecutableFallbackPython() {
+	// python3 does not exist
+	// python exists and is runnable
 	log := logging.New()
-	inspector := NewPythonInspector(util.Path{}, util.Path{}, log)
+	executor := &mockPythonExecutor{}
+	executor.On("runPythonCommand", "/some/python", mock.Anything).Return(nil, nil)
+	inspector := &defaultPythonInspector{
+		executor: executor,
+		log:      log,
+	}
 
 	exec := util.NewMockPathLooker()
 	exec.On("LookPath", "python3").Return("", os.ErrNotExist)
-	exec.On("LookPath", "python").Return("/usr/bin/python", nil)
+	exec.On("LookPath", "python").Return("/some/python", nil)
+	executable, err := inspector.getPythonExecutable(exec)
+	s.NoError(err)
+	s.Equal("/some/python", executable)
+}
+
+func (s *PythonSuite) TestGetPythonExecutablePython3NotRunnable() {
+	// python3 exists but is not runnable
+	// python exists and is runnable
+	log := logging.New()
+	executor := &mockPythonExecutor{}
+	testError := errors.New("exit status 9009")
+	executor.On("runPythonCommand", "/some/python3", mock.Anything).Return(nil, testError)
+	executor.On("runPythonCommand", "/some/python", mock.Anything).Return(nil, nil)
+
+	inspector := &defaultPythonInspector{
+		executor: executor,
+		log:      log,
+	}
+
+	exec := util.NewMockPathLooker()
+	exec.On("LookPath", "python3").Return("/some/python3", nil)
+	exec.On("LookPath", "python").Return("/some/python", nil)
 	executable, err := inspector.getPythonExecutable(exec)
 	s.NoError(err)
-	s.Equal("/usr/bin/python", executable)
+	s.Equal("/some/python", executable)
+}
+
+func (s *PythonSuite) TestGetPythonExecutableNoRunnablePython() {
+	// python3 exists but is not runnable
+	// python exists but is not runnable
+	log := logging.New()
+	executor := &mockPythonExecutor{}
+	testError := errors.New("exit status 9009")
+	executor.On("runPythonCommand", "/some/python3", mock.Anything).Return(nil, testError)
+	executor.On("runPythonCommand", "/some/python", mock.Anything).Return(nil, testError)
+
+	inspector := &defaultPythonInspector{
+		executor: executor,
+		log:      log,
+	}
+
+	exec := util.NewMockPathLooker()
+	exec.On("LookPath", "python3").Return("/some/python3", nil)
+	exec.On("LookPath", "python").Return("/some/python", nil)
+	executable, err := inspector.getPythonExecutable(exec)
+	s.NotNil(err)
+	s.Equal("", executable)
 }
 
 func (s *PythonSuite) TestGetRequirementsFromFile() {