bpo-16355: Clarify when inspect.getcomments() returns None (#428)

Initial patch by Vajrasky Kok.

Doc/library/inspect.rst

@@ -442,7 +442,9 @@ Retrieving source code
 
    Return in a single string any lines of comments immediately preceding the
    object's source code (for a class, function, or method), or at the top of the
-   Python source file (if the object is a module).
+   Python source file (if the object is a module).  If the object's source code
+   is unavailable, return ``None``.  This could happen if the object has been
+   defined in C or the interactive shell.
 
 
 .. function:: getfile(object)

Lib/test/test_inspect.py

@@ -384,6 +384,11 @@ class TestRetrievingSourceCode(GetSourceBase):
     def test_getcomments(self):
         self.assertEqual(inspect.getcomments(mod), '# line 1\n')
         self.assertEqual(inspect.getcomments(mod.StupidGit), '# line 20\n')
+        # If the object source file is not available, return None.
+        co = compile('x=1', '_non_existing_filename.py', 'exec')
+        self.assertIsNone(inspect.getcomments(co))
+        # If the object has been defined in C, return None.
+        self.assertIsNone(inspect.getcomments(list))
 
     def test_getmodule(self):
         # Check actual module