Make from_dict extensible2.19.2

Some of our consumers define additional members on the context class
that they want included in to_dict and from_dict.  While it is
possible to do this today via overrides of those functions, the
from_dict implementation in particular is a little non-obvious.
This has led to bugs when the base class's to_dict behavior changes.

This change moves the logic around extending the keys recognized by
from_dict into from_dict itself and allows consumers to simply
provide a list of those keys by overriding a class member.

Change-Id: Ib143f8a5c129dbf6711800c4d87c8830a8aa3365
Related-Bug: 1721432

2 files changed, 30 insertions, 0 deletions

diff --git a/oslo_context/context.py b/oslo_context/context.py
index 06f981e..24ded43 100644
--- a/oslo_context/context.py
+++ b/oslo_context/context.py
@@ -180,6 +180,9 @@ class RequestContext(object):
     """
 
     user_idt_format = u'{user} {tenant} {domain} {user_domain} {p_domain}'
+    # Can be overridden in subclasses to specify extra keys that should be
+    # read when constructing a context using from_dict.
+    FROM_DICT_EXTRA_KEYS = []
 
     @_renamed_kwarg('user', 'user_id')
     @_renamed_kwarg('tenant', 'project_id')
@@ -391,6 +394,8 @@ class RequestContext(object):
                           values.get('project_domain_name'))
         kwargs.setdefault('is_admin_project',
                           values.get('is_admin_project', True))
+        for key in cls.FROM_DICT_EXTRA_KEYS:
+            kwargs.setdefault(key, values.get(key))
         return cls(**kwargs)
 
     @classmethod
diff --git a/oslo_context/tests/test_context.py b/oslo_context/tests/test_context.py
index 8595975..f81f4f0 100644
--- a/oslo_context/tests/test_context.py
+++ b/oslo_context/tests/test_context.py
@@ -54,6 +54,24 @@ class Object(object):
     pass
 
 
+class TestContext(context.RequestContext):
+    """A test context with additional members
+
+    This is representative of how at least some of our consumers use the
+    RequestContext class in their projects.
+    """
+    FROM_DICT_EXTRA_KEYS = ['foo']
+
+    def __init__(self, foo=None, **kwargs):
+        super(TestContext, self).__init__(**kwargs)
+        self.foo = foo
+
+    def to_dict(self):
+        d = super(TestContext, self).to_dict()
+        d['foo'] = self.foo
+        return d
+
+
 class ContextTest(test_base.BaseTestCase):
 
     def setUp(self):
@@ -182,6 +200,13 @@ class ContextTest(test_base.BaseTestCase):
         self.assertFalse(ctx.is_admin)
         self.assertTrue(ctx.read_only)
 
+    def test_from_dict_extended(self):
+        initial = TestContext(foo='bar')
+        dct = initial.to_dict()
+        final = TestContext.from_dict(dct)
+        self.assertEqual('bar', final.foo)
+        self.assertEqual(dct, final.to_dict())
+
     def test_is_user_context(self):
         self.assertFalse(context.is_user_context(None))
         ctx = context.RequestContext(is_admin=True)