Add and use a callback name fetching utility function

Perform a better and more uniform (calling into a single
function) job at fetching a callbacks fully qualified name.

Change-Id: Ia0693b8e06fa5bed2beabfff9c8b6023e8b58980

automaton/_utils.py

@@ -0,0 +1,48 @@
+# -*- coding: utf-8 -*-
+
+#    Copyright (C) 2015 Yahoo! Inc. All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import inspect
+
+
+def get_callback_name(cb):
+    """Tries to get a callbacks fully-qualified name.
+
+    If no name can be produced ``repr(cb)`` is called and returned.
+    """
+    segments = []
+    try:
+        segments.append(cb.__qualname__)
+    except AttributeError:
+        try:
+            segments.append(cb.__name__)
+            if inspect.ismethod(cb):
+                try:
+                    # This attribute doesn't exist on py3.x or newer, so
+                    # we optionally ignore it... (on those versions of
+                    # python `__qualname__` should have been found anyway).
+                    segments.insert(0, cb.im_class.__name__)
+                except AttributeError:
+                    pass
+        except AttributeError:
+            pass
+    if not segments:
+        return repr(cb)
+    else:
+        try:
+            segments.insert(0, cb.__module__)
+        except AttributeError:
+            pass
+        return ".".join(segments)

automaton/machines.py

@@ -25,6 +25,7 @@ from debtcollector import removals
 import prettytable
 import six
 
+from automaton import _utils as utils
 from automaton import exceptions as excp
 
 
@@ -326,35 +327,23 @@ class FiniteMachine(object):
                     target = self._transitions[state][event]
                     row = [pretty_state, event, target.name]
                     if target.on_enter is not None:
-                        try:
-                            row.append(target.on_enter.__name__)
-                        except AttributeError:
-                            row.append(target.on_enter)
+                        row.append(utils.get_callback_name(target.on_enter))
                     else:
                         row.append(empty)
                     if target.on_exit is not None:
-                        try:
-                            row.append(target.on_exit.__name__)
-                        except AttributeError:
-                            row.append(target.on_exit)
+                        row.append(utils.get_callback_name(target.on_exit))
                     else:
                         row.append(empty)
                     tbl.add_row(row)
             else:
                 on_enter = self._states[state]['on_enter']
                 if on_enter is not None:
-                    try:
-                        on_enter = on_enter.__name__
-                    except AttributeError:
-                        pass
+                    on_enter = utils.get_callback_name(on_enter)
                 else:
                     on_enter = empty
                 on_exit = self._states[state]['on_exit']
                 if on_exit is not None:
-                    try:
-                        on_exit = on_exit.__name__
-                    except AttributeError:
-                        pass
+                    on_exit = utils.get_callback_name(on_exit)
                 else:
                     on_exit = empty
                 tbl.add_row([pretty_state, empty, empty, on_enter, on_exit])