Add the command line to the AssertionError raised by test.script_helper's

Python subprocess failure assertion error messages for easier debugging.

Adds a unittest for test.script_helper to confirm that this code works as
it is otherwise uncovered by an already passing test suite that uses it. :)

Lib/test/script_helper.py

@@ -51,8 +51,9 @@ def _assert_python(expected_success, *args, **env_vars):
     err = strip_python_stderr(err)
     if (rc and expected_success) or (not rc and not expected_success):
         raise AssertionError(
-            "Process return code is %d, "
-            "stderr follows:\n%s" % (rc, err.decode('ascii', 'ignore')))
+            "Process return code is %d, command line was: %r, "
+            "stderr follows:\n%s" % (rc, cmd_line,
+                                     err.decode('ascii', 'ignore')))
     return rc, out, err
 
 def assert_python_ok(*args, **env_vars):

Lib/test/test_script_helper.py

@@ -0,0 +1,35 @@
+"""Unittests for test.script_helper.  Who tests the test helper?"""
+
+from test import script_helper
+import unittest
+
+
+class TestScriptHelper(unittest.TestCase):
+    def test_assert_python_expect_success(self):
+        t = script_helper._assert_python(True, '-c', 'import sys; sys.exit(0)')
+        self.assertEqual(0, t[0], 'return code was not 0')
+
+    def test_assert_python_expect_failure(self):
+        # I didn't import the sys module so this child will fail.
+        rc, out, err = script_helper._assert_python(False, '-c', 'sys.exit(0)')
+        self.assertNotEqual(0, rc, 'return code should not be 0')
+
+    def test_assert_python_raises_expect_success(self):
+        # I didn't import the sys module so this child will fail.
+        with self.assertRaises(AssertionError) as error_context:
+            script_helper._assert_python(True, '-c', 'sys.exit(0)')
+        error_msg = str(error_context.exception)
+        self.assertIn('command line was:', error_msg)
+        self.assertIn('sys.exit(0)', error_msg, msg='unexpected command line')
+
+    def test_assert_python_raises_expect_failure(self):
+        with self.assertRaises(AssertionError) as error_context:
+            script_helper._assert_python(False, '-c', 'import sys; sys.exit(0)')
+        error_msg = str(error_context.exception)
+        self.assertIn('Process return code is 0,', error_msg)
+        self.assertIn('import sys; sys.exit(0)', error_msg,
+                      msg='unexpected command line.')
+
+
+if __name__ == '__main__':
+    unittest.main()