Using single-dispatch generic functions (PEP 443) to implement an extensible JSON encoder To use with Python 2.6 to 3.3, install singledispatch from PyPI.

example.py

from decimal import Decimal

from json_singledispatch import encode


@encode.register(set)
def encode_set(obj):
    return encode(list(obj))


@encode.register(Decimal)
def encode_decimal(obj):
    return encode(str(obj))


print encode({'key': "value"})
print encode({5, 6})
print encode(Decimal("5.6"))

json_singledispatch.py

import json
from singledispatch import singledispatch


class _CustomEncoder(json.JSONEncoder):
    def default(self, obj):
        for type_, handler in encode.registry.items():
            if isinstance(obj, type_) and type_ is not object:
                return handler(obj)
        return super(_CustomEncoder, self).default(obj)


@singledispatch
def encode(obj, **kwargs):
    return json.dumps(obj, cls=_CustomEncoder, **kwargs)