Add a registry that keeps track of tag->renderer/input mappings

common/lib/capa/capa/registry.py

+class TagRegistry(object):
+    """
+    A registry mapping tags to handlers.
+
+    (A dictionary with some extra error checking.)
+    """
+    def __init__(self):
+        self._mapping = {}
+
+    def register(self, cls):
+        """
+        Register cls as a supported tag type.  It is expected to define cls.tags as a list of tags
+        that it implements.
+
+        If an already-registered type has registered one of those tags, will raise ValueError.
+
+        If there are no tags in cls.tags, will also raise ValueError.
+        """
+
+        # Do all checks and complain before changing any state.
+        if len(cls.tags) == 0:
+            raise ValueError("No tags specified for class {0}".format(cls.__name__))
+
+        for t in cls.tags:
+            if t in self._mapping:
+                other_cls = self._mapping[t]
+                if cls == other_cls:
+                    # registering the same class multiple times seems silly, but ok
+                    continue
+                raise ValueError("Tag {0} already registered by class {1}."
+                                 " Can't register for class {2}"
+                                     .format(t, other_cls.__name__, cls.__name__))
+
+        # Ok, should be good to change state now.
+        for t in cls.tags:
+            self._mapping[t] = cls
+
+    def registered_tags(self):
+        """
+        Get a list of all the tags that have been registered.
+        """
+        return self._mapping.keys()
+
+    def get_class_for_tag(self, tag):
+        """
+        For any tag in registered_tags(), returns the corresponding class.  Otherwise, will raise
+        KeyError.
+        """
+        return self._mapping[tag]