See nedbat#33

Because instances of Falsy are falsy, `func.im_self` in
call_function() of xpython/pyvm2.ptreated them as nonexistent and
did not add the self parameter to the argument list.

The problem was exhibited on Python 2.7.

Author: rocky

test/bytecode-2.5/test_bound_method_on_falsy_objects.pyc

@@ -0,0 +1,15 @@
+# See https://github.com/nedbat/byterun/pull/33:
+#
+# Because instances of Falsy are falsy, `func.im_self` in
+# call_function() of xpython/pyvm2.ptreated them as nonexistent and
+# did not add the self parameter to the argument list.
+#
+# The problem was exhibited on Python 2.7.
+class Falsy(object):
+    def __bool__(self):
+            return False
+    __nonzero__ = __bool__
+    def do_stuff(self):
+            pass
+
+Falsy().do_stuff()

test/bytecode-2.6/test_bound_method_on_falsy_objects.pyc

@@ -26,6 +26,9 @@ def test_globals(self):
     def test_inplace_operators(self):
         self.do_one()
 
+    def test_bound_method_on_falsy_objects(self):
+        self.do_one()
+
     if PY2:
         def test_inplace_division(self):
             self.assert_ok("""\

test/bytecode-2.7/test_bound_method_on_falsy_objects.pyc

@@ -774,7 +774,7 @@ def call_function(self, arg, args, kwargs):
         func = self.pop()
         if hasattr(func, "im_func"):
             # Methods get self as an implicit first parameter.
-            if func.im_self:
+            if func.im_self is not None:
                 posargs.insert(0, func.im_self)
             # The first parameter must be the correct type.
             if not isinstance(posargs[0], func.im_class):