Implement set/get model constraints (juju#253)

* Implement set/get model constraints

Add methods for setting and getting model constraints.

* Fix model constraint name in tests

juju/model.py

@@ -22,6 +22,7 @@
 from . import tag, utils
 from .client import client, connector
 from .client.client import ConfigValue
+from .client.client import Value
 from .constraints import parse as parse_constraints
 from .constraints import normalize_key
 from .delta import get_entity_class, get_entity_delta
@@ -1461,11 +1462,28 @@ async def get_config(self):
             config[key] = ConfigValue.from_json(value)
         return config
 
-    def get_constraints(self):
+    async def get_constraints(self):
         """Return the machine constraints for this model.
 
+        :returns: A ``dict`` of constraints.
         """
-        raise NotImplementedError()
+        constraints = {}
+        client_facade = client.ClientFacade.from_connection(self.connection())
+        result = await client_facade.GetModelConstraints()
+
+        # GetModelConstraints returns GetConstraintsResults which has a 'constraints'
+        # attribute. If no constraints have been set GetConstraintsResults.constraints
+        # is None. Otherwise GetConstraintsResults.constraints has an attribute for each
+        # possible constraint, each of these in turn will be None if they have not been
+        # set.
+        if result.constraints:
+           constraint_types = [a for a in dir(result.constraints)
+                               if a in Value._toSchema.keys()]
+           for constraint in constraint_types:
+               value = getattr(result.constraints, constraint)
+               if value is not None:
+                   constraints[constraint] = getattr(result.constraints, constraint)
+        return constraints
 
     def import_ssh_key(self, identity):
         """Add a public SSH key from a trusted indentity source to this model.
@@ -1624,13 +1642,15 @@ async def set_config(self, config):
                 config[key] = value.value
         await config_facade.ModelSet(config)
 
-    def set_constraints(self, constraints):
+    async def set_constraints(self, constraints):
         """Set machine constraints on this model.
 
-        :param :class:`juju.Constraints` constraints: Machine constraints
-
+        :param dict config: Mapping of model constraints
         """
-        raise NotImplementedError()
+        client_facade = client.ClientFacade.from_connection(self.connection())
+        await client_facade.SetModelConstraints(
+            application='',
+            constraints=constraints)
 
     async def get_action_output(self, action_uuid, wait=None):
         """Get the results of an action by ID.

tests/integration/test_model.py

@@ -392,6 +392,14 @@ async def test_config(event_loop):
         assert result['extra-info'].source == 'model'
         assert result['extra-info'].value == 'booyah'
 
+@base.bootstrapped
+@pytest.mark.asyncio
+async def test_set_constraints(event_loop):
+    async with base.CleanModel() as model:
+        await model.set_constraints({'cpu-power': 1})
+        cons = await model.get_constraints()
+        assert cons['cpu_power'] == 1
+
 # @base.bootstrapped
 # @pytest.mark.asyncio
 # async def test_grant(event_loop)