Make dynamic attributes of model introspectable

`dir(object)` is a useful Python function that tells you what attributes
and methods an object has. It’s also used by tools like iPython and IDEs
for code completion.

Some of the attributes of a `JSONModel` are dynamic, based on what
fields we expect in the underlying JSON. Therefore they don’t
automatically end up in the result of calling `dir`. To get around this
we can implement our own `__dir__` method, which also returns the names
of the fields we’re expecting the the JSON.

Inspired by this Raymond Hettinger tweet:

> #python tip:  If you add attributes to an API with __getattr__() or
> __getattribute__(), remember to update __dir__() to make the extension
> introspectable.

— https://twitter.com/raymondh/status/1249860863525146624

app/models/__init__.py

@@ -18,6 +18,9 @@ class JSONModel():
     def __hash__(self):
         return hash(self.id)
 
+    def __dir__(self):
+        return super().__dir__() + list(sorted(self.ALLOWED_PROPERTIES))
+
     def __eq__(self, other):
         return self.id == other.id
 

tests/app/models/test_base_model.py

@@ -68,3 +68,11 @@ def test_model_doesnt_swallow_attribute_errors(json_response):
         Custom(json_response).foo
 
     assert str(e.value) == 'Something has gone wrong'
+
+
+def test_dynamic_properties_are_introspectable():
+
+    class Custom(JSONModel):
+        ALLOWED_PROPERTIES = {'foo', 'bar', 'baz'}
+
+    assert dir(Custom({}))[-3:] == ['bar', 'baz', 'foo']