Add a register_interface_if decorator. (#3120)

* Add a register_interface_if decorator.

* Add tests.

* PEP 8.

1 files changed, 27 insertions, 1 deletions

diff --git a/tests/test_interfaces.py b/tests/test_interfaces.py
index bdb4a94d..97df45a3 100644
--- a/tests/test_interfaces.py
+++ b/tests/test_interfaces.py
@@ -8,7 +8,33 @@ import pytest
 
 import six
 
-from cryptography.utils import InterfaceNotImplemented, verify_interface
+from cryptography.utils import (
+    InterfaceNotImplemented, register_interface_if, verify_interface
+)
+
+
+def test_register_interface_if_true():
+    @six.add_metaclass(abc.ABCMeta)
+    class SimpleInterface(object):
+        pass
+
+    @register_interface_if(1 == 1, SimpleInterface)
+    class SimpleClass(object):
+        pass
+
+    assert issubclass(SimpleClass, SimpleInterface) is True
+
+
+def test_register_interface_if_false():
+    @six.add_metaclass(abc.ABCMeta)
+    class SimpleInterface(object):
+        pass
+
+    @register_interface_if(1 == 2, SimpleInterface)
+    class SimpleClass(object):
+        pass
+
+    assert issubclass(SimpleClass, SimpleInterface) is False
 
 
 class TestVerifyInterface(object):