Inheriting the class dictionary with metaclasses

Question

I am setting a class property fields using a metaclass:

class MyMeta(type):
    def __new__(mcs, name, bases, clsdict):

        clsdict['fields'] = {k: v
                             for k, v in clsdict.items()
                             if <my_condition>}
        return super(MyMeta, mcs).__new__(mcs, name, bases, clsdict)

class MyBaseClass(metaclass=MyMeta):
    fields = {}

The following instantiation leads to expected results:

class SubClass(MyBaseClass):
    param1 = 1 # meets <my_condition>

>>> SubClass.fields
{param1: 1}

But if I now subclass SubClass, fields is empty:

class SubSubClass(SubClass):
   pass

>>> SubSubClass.fields 
{}

How would I be able to update the classdict of all classes in inheritance hierarchy so that the fields variable would be updated from base classes?

Answer 1

You need to somehow keep the fields of the superclasses, for example by iterating over the "bases" and using their fields as starting point:

class MyMeta(type):
    def __new__(mcs, name, bases, clsdict):
        if 'fields' not in clsdict:
            clsdict['fields'] = {}
        # Initialize "fields" from base classes
        for base in bases:
            try:
                clsdict['fields'].update(base.fields)
            except AttributeError:
                pass
        # Fill in new fields (I included a "trivial" condition here, just use yours instead.)
        clsdict['fields'].update({k: v for k, v in clsdict.items() if k.startswith('param')})
        return super(MyMeta, mcs).__new__(mcs, name, bases, clsdict)

And it works for SubClass and SubSubClass:

>>> SubClass.fields
{'param1': 1}

>>> SubSubClass.fields
{'param1': 1}