Python 3 support for classmethod call targets

There were two problems with detection and handling of class methods as call targets in Python 3:

* Methods no longer define `im_self` -- this is now only `__self__`
* The `types` module no longer defines a `ClassType`

The universally-compatible (v2.6+) solution was to switch to using the `inspect` module -- whose interface has been stable -- and to checking the method attribute `__self__`.

(It doesn't hurt that `inspect` checks are more brief and we now no longer need the `types` module here.)

mitogen/parent.py

@@ -47,7 +47,6 @@ import termios
 import textwrap
 import threading
 import time
-import types
 import zlib
 
 # Absolute imports for <2.5.
@@ -490,9 +489,8 @@ def upgrade_router(econtext):
 
 
 def make_call_msg(fn, *args, **kwargs):
-    if isinstance(fn, types.MethodType) and \
-       isinstance(fn.im_self, (type, types.ClassType)):
-        klass = mitogen.core.to_text(fn.im_self.__name__)
+    if inspect.ismethod(fn) and inspect.isclass(fn.__self__):
+        klass = mitogen.core.to_text(fn.__self__.__name__)
     else:
         klass = None
 

tests/call_function_test.py

@@ -36,7 +36,17 @@ def func_accepts_returns_sender(sender):
     return sender
 
 
+class TargetClass:
+
+    offset = 100
+
+    @classmethod
+    def add_numbers_with_offset(cls, x, y):
+        return cls.offset + x + y
+
+
 class CallFunctionTest(testlib.RouterMixin, testlib.TestCase):
+
     def setUp(self):
         super(CallFunctionTest, self).setUp()
         self.local = self.router.fork()
@@ -44,6 +54,12 @@ class CallFunctionTest(testlib.RouterMixin, testlib.TestCase):
     def test_succeeds(self):
         self.assertEqual(3, self.local.call(function_that_adds_numbers, 1, 2))
 
+    def test_succeeds_class_method(self):
+        self.assertEqual(
+            self.local.call(TargetClass.add_numbers_with_offset, 1, 2),
+            103,
+        )
+
     def test_crashes(self):
         exc = self.assertRaises(mitogen.core.CallError,
             lambda: self.local.call(function_that_fails))