refactor: replace _AutoNamedFieldsMetaclass with __set_name__

_AutoNamedFieldsMetaclass was a hand-rolled way of capturing each
Field's name as it is defined on an XBlock. Since Python 3.6,
__set_name__ is part of the built-in Descriptor interface, providing
a more elegant way of achieving the same thing.

Author: kdmccormick

xblock/core.py

@@ -38,47 +38,7 @@
 UNSET = object()
 
 
-class _AutoNamedFieldsMetaclass(type):
-    """
-    Builds classes such that their Field attributes know their own names.
-
-    This allows XBlock API users to define fields without name redundancy, e.g. like this:
-
-        class MyBlock(XBlock):
-            my_field = Field(...)
-
-    rather than this:
-
-        class MyBlock(XBlock):
-            my_field = Field(name="my_field", ...)
-    """
-    def __new__(mcs, name, bases, attrs):
-        """
-        Ensure __name__ is set on all Field attributes, both on the new class and on its bases.
-        """
-        def needs_name(obj):
-            """
-            Is this object a Field that hasn't had its name assigned yet?
-            """
-            return isinstance(obj, Field) and not obj.__name__
-
-        # Iterate over the attrs before they're bound to the class
-        # so that we don't accidentally trigger any __get__ methods
-        for attr_name, attr in attrs.items():
-            if needs_name(attr):
-                attr.__name__ = attr_name
-
-        # Iterate over all of the base classes, so that we can add
-        # names to any mixins that don't include this metaclass, but that
-        # do include Fields
-        for base in bases:
-            for attr_name, attr in inspect.getmembers(base, predicate=needs_name):
-                attr.__name__ = attr_name
-
-        return super().__new__(mcs, name, bases, attrs)
-
-
-class Blocklike(metaclass=_AutoNamedFieldsMetaclass):
+class Blocklike:
     """
     Shared base for XBlocks and XBlockAsides, providing these common capabilities:
 
@@ -645,7 +605,7 @@ class XBlockMixin(Blocklike):
     """
 
 
-class _HasChildrenMetaclass(_AutoNamedFieldsMetaclass):
+class _HasChildrenMetaclass(type):
     """
     Adds a ``children`` XBlock ReferenceList field to classes where ``has_children == True``.
     """

xblock/fields.py

@@ -331,6 +331,12 @@ def __init__(self, help=None, default=UNSET, scope=Scope.content,  # pylint:disa
         self.xml_node = xml_node
         self.force_export = force_export
 
+    def __set_name__(self, owner: type, name: str):
+        """
+        Automatically record the name of this Field as it is defined on a class.
+        """
+        self.__name__ = name
+
     @property
     def default(self):
         """Returns the static value that this defaults to."""