Add update_current_context to ContextMixin

 - `update_current_context` allows changing of a context without
   entering a `with` block.
 - Corrects `Context.__init__` to copy the dictionary it is passed, not
   doing so caused some bugs when introducing `update_current_context`.

rig/utils/contexts.py

@@ -67,6 +67,10 @@ def get_new_context(self, **kwargs):
         """Create a new context with the given keyword arguments."""
         return Context(kwargs, self.__context_stack)
 
+    def update_current_context(self, **context_args):
+        """Update the current context to contain new arguments."""
+        self.__context_stack[-1].update(context_args)
+
     def get_context_arguments(self):
         """Return a dictionary containing the current context arguments."""
         cargs = {}
@@ -161,9 +165,13 @@ def __init__(self, context_arguments, stack=None):
             Context stack to which this context will append itself when
             entered.
         """
-        self.context_arguments = context_arguments
+        self.context_arguments = dict(context_arguments)
         self.stack = stack
 
+    def update(self, updates):
+        """Update the arguments contained within this context."""
+        self.context_arguments.update(updates)
+
     def __enter__(self):
         # Add this context object to the stack
         self.stack.append(self)

rig/utils/tests/test_contexts.py

@@ -63,6 +63,43 @@ def test_contextmixin_required_passed(object_to_test, arg1, arg2):
             (123, "Hello", 4, tuple(), {})
 
 
+@pytest.mark.parametrize("arg1", [1, None, 5])
+@pytest.mark.parametrize("arg2", [1, None, 5])
+def test_contextmixin_update_current_context(object_to_test, arg1, arg2):
+    # Create the object
+    obj = object_to_test()
+
+    # Missing arguments currently
+    with pytest.raises(TypeError) as excinfo:
+        obj.method_a(1)
+    assert "arg1" in str(excinfo.value)
+    assert "method_a" in str(excinfo.value)
+
+    # Update the current context
+    obj.update_current_context(arg1=arg1, bob=3)
+
+    assert obj.method_a(1) == (1, arg1, 30)
+    assert obj.method_a(1, arg2=50) == (1, arg1, 50)
+
+    # And again
+    obj.update_current_context(arg2=arg2)
+    assert (obj.method_b("World", "Hello") ==
+            ("World", arg1, arg2, ("Hello", ), {}))
+    assert (obj.method_b(123, arg1="Hello", arg2=4) ==
+            (123, "Hello", 4, tuple(), {}))
+
+    # Within a context
+    t_arg1 = 11111
+    with obj.get_new_context(arg1=t_arg1):
+        assert obj.method_a(1) == (1, t_arg1, arg2)
+
+        obj.update_current_context(arg1=arg1)
+        assert obj.method_a(1) == (1, arg1, arg2)
+
+    # And outside of that context
+    assert obj.method_a(1) == (1, arg1, arg2)
+
+
 @pytest.mark.parametrize("arg1", [1, None, 5])
 def test_contextmixin_required_not_passed_context(object_to_test, arg1):
     # Create the object