Is it possible to serializing UUID keys in a Python dictionary

Question

In Python, the json module allows for custom types to be serialized via either the cls or default named arguments. However, this does not seem to work for the keys in a dictionary as per the following code snippet.

import json
import uuid

# works
mydict = {
    'helo': uuid.uuid4(),
    'world': uuid.uuid4(),
}
print(json.dumps(mydict, default=str))

# raises TypeError: keys must be str, int, float, bool or None, not UUID
mydict = {
    uuid.uuid4(): 'helo',
    uuid.uuid4(): 'world',
}
print(json.dumps(mydict, default=str))

This is an oversight? A bug? Why would default not also be used as a fallback method for serializing the keys?