Issue #14285: Do not catch __init__.py exceptions in runpy

Initialize package before calling find_spec() for __main__, so that we do not
incorrectly handle exceptions from __init__.py. When runpy is used from the
Python CLI, use an internal exception rather than ImportError, to avoid
catching unexpected exceptions.

Also remove exception message rewriting in _run_module_as_main(), because it
seems to be redundant with the _get_main_module_details() function.

Lib/test/test_cmd_line_script.py

@@ -397,20 +397,73 @@ class CmdLineTest(unittest.TestCase):
                                       script_name, script_name, '', '',
                                       importlib.machinery.SourceFileLoader)
 
+    @contextlib.contextmanager
+    def setup_test_pkg(self, *args):
+        with support.temp_dir() as script_dir, \
+                support.change_cwd(path=script_dir):
+            pkg_dir = os.path.join(script_dir, 'test_pkg')
+            make_pkg(pkg_dir, *args)
+            yield pkg_dir
+
+    def check_dash_m_failure(self, *args):
+        rc, out, err = assert_python_failure('-m', *args, __isolated=False)
+        if verbose > 1:
+            print(repr(out))
+        self.assertEqual(rc, 1)
+        return err
+
     def test_dash_m_error_code_is_one(self):
         # If a module is invoked with the -m command line flag
         # and results in an error that the return code to the
         # shell is '1'
-        with support.temp_dir() as script_dir:
-            with support.change_cwd(path=script_dir):
-                pkg_dir = os.path.join(script_dir, 'test_pkg')
-                make_pkg(pkg_dir)
-                script_name = _make_test_script(pkg_dir, 'other',
-                                                "if __name__ == '__main__': raise ValueError")
-                rc, out, err = assert_python_failure('-m', 'test_pkg.other', *example_args)
-                if verbose > 1:
-                    print(repr(out))
+        with self.setup_test_pkg() as pkg_dir:
+            script_name = _make_test_script(pkg_dir, 'other',
+                                            "if __name__ == '__main__': raise ValueError")
+            err = self.check_dash_m_failure('test_pkg.other', *example_args)
+            self.assertIn(b'ValueError', err)
+
+    def test_dash_m_errors(self):
+        # Exercise error reporting for various invalid package executions
+        tests = (
+            ('builtins', br'No code object available'),
+            ('builtins.x', br'Error while finding spec.*AttributeError'),
+            ('builtins.x.y', br'Error while finding spec.*'
+                br'ImportError.*No module named.*not a package'),
+            ('os.path', br'loader.*cannot handle'),
+            ('importlib', br'No module named.*'
+                br'is a package and cannot be directly executed'),
+            ('importlib.nonexistant', br'No module named'),
+        )
+        for name, regex in tests:
+            with self.subTest(name):
+                rc, _, err = assert_python_failure('-m', name)
                 self.assertEqual(rc, 1)
+                self.assertRegex(err, regex)
+                self.assertNotIn(b'Traceback', err)
+
+    def test_dash_m_init_traceback(self):
+        # These were wrapped in an ImportError and tracebacks were
+        # suppressed; see Issue 14285
+        exceptions = (ImportError, AttributeError, TypeError, ValueError)
+        for exception in exceptions:
+            exception = exception.__name__
+            init = "raise {0}('Exception in __init__.py')".format(exception)
+            with self.subTest(exception), \
+                    self.setup_test_pkg(init) as pkg_dir:
+                err = self.check_dash_m_failure('test_pkg')
+                self.assertIn(exception.encode('ascii'), err)
+                self.assertIn(b'Exception in __init__.py', err)
+                self.assertIn(b'Traceback', err)
+
+    def test_dash_m_main_traceback(self):
+        # Ensure that an ImportError's traceback is reported
+        with self.setup_test_pkg() as pkg_dir:
+            main = "raise ImportError('Exception in __main__ module')"
+            _make_test_script(pkg_dir, '__main__', main)
+            err = self.check_dash_m_failure('test_pkg')
+            self.assertIn(b'ImportError', err)
+            self.assertIn(b'Exception in __main__ module', err)
+            self.assertIn(b'Traceback', err)
 
     def test_pep_409_verbiage(self):
         # Make sure PEP 409 syntax properly suppresses