In ACMEv2, challenges have "url" instead of "uri". To handle this smoothly, Challenge's uri field becomes private (_uri), and is joined by _url. Serialization and deserialization will preserve whichever one was set. The uri name is taken over by an @property that returns whichever of the two is set. I chose not to enforce that they shouldn't both be present because it would just add unnecessary code and brittleness with no stability benefit.

* Make url a virtual field.

* Add @property annotation.

acme/acme/client_test.py

@@ -181,7 +181,7 @@ class ClientTest(unittest.TestCase):
 
         # TODO: split here and separate test
         self.assertRaises(errors.UnexpectedUpdate, self.client.answer_challenge,
-                          self.challr.body.update(uri='foo'), chall_response)
+                          self.challr.body.update(_uri='foo'), chall_response)
 
     def test_answer_challenge_missing_next(self):
         self.assertRaises(

acme/acme/messages.py

@@ -325,13 +325,26 @@ class ChallengeBody(ResourceBody):
 
     """
     __slots__ = ('chall',)
-    uri = jose.Field('uri')
+    # ACMEv1 has a "uri" field in challenges. ACMEv2 has a "url" field. This
+    # challenge object supports either one. In Client.answer_challenge,
+    # whichever one is set will be used.
+    _uri = jose.Field('uri', omitempty=True, default=None)
+    _url = jose.Field('url', omitempty=True, default=None)
     status = jose.Field('status', decoder=Status.from_json,
                         omitempty=True, default=STATUS_PENDING)
     validated = fields.RFC3339Field('validated', omitempty=True)
     error = jose.Field('error', decoder=Error.from_json,
                        omitempty=True, default=None)
 
+    def __init__(self, **kwargs):
+        new_kwargs = {}
+        for k, v in kwargs.items():
+            if k in ('uri', 'url',):
+                k = '_' + k
+            new_kwargs[k] = v
+        # pylint: disable=star-args
+        super(ChallengeBody, self).__init__(**new_kwargs)
+
     def to_partial_json(self):
         jobj = super(ChallengeBody, self).to_partial_json()
         jobj.update(self.chall.to_partial_json())
@@ -343,6 +356,11 @@ class ChallengeBody(ResourceBody):
         jobj_fields['chall'] = challenges.Challenge.from_json(jobj)
         return jobj_fields
 
+    @property
+    def uri(self):
+        """The URL of this challenge."""
+        return self._url or self._uri
+
     def __getattr__(self, name):
         return getattr(self.chall, name)
 
@@ -358,10 +376,10 @@ class ChallengeResource(Resource):
     authzr_uri = jose.Field('authzr_uri')
 
     @property
-    def uri(self):  # pylint: disable=missing-docstring,no-self-argument
-        # bug? 'method already defined line None'
-        # pylint: disable=function-redefined
-        return self.body.uri  # pylint: disable=no-member
+    def uri(self):
+        """The URL of the challenge body."""
+        # pylint: disable=function-redefined,no-member
+        return self.body.uri
 
 
 class Authorization(ResourceBody):