json.dumps on dictionary with bytes for keys

you could pre-process the dictionary to convert the keys as strings recursively if they're bytes

import json
# your dump code for values, unmodified
class BytesDump(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, bytes):
            return obj.decode()
        return json.JSONEncoder.default(self, obj)

# recursive key as string conversion for byte keys
def keys_string(d):
    rval = {}
    if not isinstance(d, dict):
        if isinstance(d,(tuple,list,set)):
            v = [keys_string(x) for x in d]
            return v
        else:
            return d

    for k,v in d.items():
        if isinstance(k,bytes):
            k = k.decode()
        if isinstance(v,dict):
            v = keys_string(v)
        elif isinstance(v,(tuple,list,set)):
            v = [keys_string(x) for x in v]
        rval[k] = v
    return rval

print(json.dumps(keys_string(bar), cls=BytesDump))

with:

bar = {b'name': b'bob', b'age': 33, b'attributes': {b'hair': b'brown', b'arms': 2},
b'other': [{b'hair': b'brown', b'arms': 2}]}

prints:

{"attributes": {"hair": "brown", "arms": 2}, "age": 33, "name": "bob", "other": [{"hair": "brown", "arms": 2}]}

Looks like you need to use a recursive utility function:

import json

def decode_dict(d):
    result = {}
    for key, value in d.items():
        if isinstance(key, bytes):
            key = key.decode()
        if isinstance(value, bytes):
            value = value.decode()
        elif isinstance(value, dict):
            value = decode_dict(value)
        result.update({key: value})
    return result


bar = {b'name': b'bob', b'age': 33, b'attributes': {b'hair': b'brown', b'arms': 2}}
print(json.dumps(decode_dict(bar)))

Output:

{"name": "bob", "age": 33, "attributes": {"hair": "brown", "arms": 2}}