From 6511ba63edc00887b4fb8ab06e89fef19e53276f Mon Sep 17 00:00:00 2001
From: Tim Mitchell <tim.mitchell@seequent.com>
Date: Wed, 22 Sep 2021 14:53:05 +1200
Subject: [PATCH] #83: Add special handling for __call__ to interface_only().

---
 pure_interface.py      |  2 ++
 tests/test_adaption.py | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/pure_interface.py b/pure_interface.py
index be09b8b..237f1a0 100644
--- a/pure_interface.py
+++ b/pure_interface.py
@@ -629,6 +629,8 @@ def interface_only(cls, implementation):
         if cls._pi.impl_wrapper_type is None:
             type_name = '_{}Only'.format(cls.__name__)
             attributes = {'__module__': cls.__module__}
+            if '__call__' in cls._pi.interface_names:
+                attributes['__call__'] = getattr(implementation, '__call__')
             cls._pi.impl_wrapper_type = type(type_name, (_ImplementationWrapper,), attributes)
             abc.ABCMeta.register(cls, cls._pi.impl_wrapper_type)
         return cls._pi.impl_wrapper_type(implementation, cls)
diff --git a/tests/test_adaption.py b/tests/test_adaption.py
index bea4d45..b53b859 100644
--- a/tests/test_adaption.py
+++ b/tests/test_adaption.py
@@ -114,6 +114,23 @@ def speak(self, volume):
         super(SleepTalker, self).speak(volume)
 
 
+class DunderInterface(pure_interface.Interface):
+    def __call__(self, a):
+        pass
+
+    def __len__(self):
+        pass
+
+
+class DunderClass(DunderInterface, object):
+
+    def __call__(self, a):
+        super().__call__(a)
+
+    def __len__(self):
+        return 5
+
+
 class TestAdaption(unittest.TestCase):
     @classmethod
     def setUpClass(cls):
@@ -350,3 +367,16 @@ def test_adapt_interface_only(self):
             ISpeaker.adapt(talker_only)
         except:
             self.fail('adaption of interface only failed.')
+
+    def test_adapt_callable_is_callable(self):
+        dunder = DunderClass()
+        dunder_only = DunderInterface.adapt(dunder)
+        try:
+            dunder_only(1)
+        except TypeError:
+            self.fail('calling interface only failed')
+
+        try:
+            len(dunder)
+        except:
+            self.fail('len() interface only failed')