Allow blocks to override Field defaults without copying the original field

[cpennington]

Currently, to change the default value of a field (such as display_name), a block needs to copy the definition. This means that if other values in the field need to change, they need to be changed in many places, which is subpar.

[cpennington]

When this happens, accidentally overriding a field should raise an exception.

Proposal:

class FooBlock(XBlock):
    display_name = OverrideField(default='Foo Block')  # Uses the same field definition as the parent, with kwargs overridden
    tags = Field(...)  # raises an exception, because it hides a parent field
    name = OverrideField(scope=scope.user_state)  # Error, can't override the scope

[cpennington]

@nedbat @jorr-at-google: Thoughts?

[nedbat]

How often has a subclass wanted to keep a field but change its default? This seems like an odd operation to me. There must be some real examples we can look at, I know you didn't invent this for the fun of it.

[cpennington]

We do it constantly for display_name.