Avoid placing Block instances into a set(), as this requires them to be hashable.

Defining Block.__eq__ in 047ff734ca causes the default __hash__ implementation to be nullified on Python 3. Since it's non-trivial to define a replacement __hash__ method that's consistent with our __eq__ behaviour (it depends on the equality of lists / dicts, which are themselves unhashable), it's more correct to explicitly define Block as unhashable, bringing the Python 2 behaviour in line with Python 3.

The one place where we currently depend on Block being hashable is in constructing the 'dependencies' set for nested blocks. Since we don't actually need the de-duplicating behaviour of set(), we can replace this with a plain list with no ill effects.

wagtail/wagtailadmin/blocks.py

@@ -77,7 +77,7 @@ class Block(six.with_metaclass(BaseBlock, object)):
     the base 'media' and 'html_declarations' methods will return those declarations; the outer block type can
     then add its own declarations to the list by overriding those methods and using super().
     """
-    dependencies = set()
+    dependencies = []
 
     def all_blocks(self):
         """
@@ -277,6 +277,12 @@ class Block(six.with_metaclass(BaseBlock, object)):
     def __ne__(self, other):
         return not self.__eq__(other)
 
+    # Making block instances hashable in a way that's consistent with __eq__ is non-trivial, because
+    # self.deconstruct() is liable to contain unhashable data (e.g. lists and dicts). So let's set
+    # Block to be explicitly unhashable - Python 3 will do this automatically when defining __eq__,
+    # but Python 2 won't, and we'd like the behaviour to be consistent on both.
+    __hash__ = None
+
 
 class BoundBlock(object):
     def __init__(self, block, value, prefix=None, errors=None):
@@ -454,7 +460,7 @@ class BaseStructBlock(Block):
             if js_initializer is not None:
                 self.child_js_initializers[name] = js_initializer
 
-        self.dependencies = set(self.child_blocks.values())
+        self.dependencies = self.child_blocks.values()
 
     def js_initializer(self):
         # skip JS setup entirely if no children have js_initializers
@@ -609,7 +615,7 @@ class ListBlock(Block):
         else:
             self.child_block = child_block
 
-        self.dependencies = set([self.child_block])
+        self.dependencies = [self.child_block]
         self.child_js_initializer = self.child_block.js_initializer()
 
     @property
@@ -747,7 +753,7 @@ class BaseStreamBlock(Block):
                 block.set_name(name)
                 self.child_blocks[name] = block
 
-        self.dependencies = set(self.child_blocks.values())
+        self.dependencies = self.child_blocks.values()
 
     def render_list_member(self, block_type_name, value, prefix, index, errors=None):
         """