You can serialise the __dict__ attribute of m:

In [214]: json.dumps(m.__dict__)
Out[214]: '{"quadruple": 400, "double": 200, "triple": 300}'

You can also call vars:

In [216]: json.dumps(vars(m))
Out[216]: '{"quadruple": 400, "double": 200, "triple": 300}'

What to use and why: Use `__dict__` or `vars()`?

For more complicated classes, consider the use of jsonpickle.

jsonpickle is a Python library for serialization and deserialization of complex Python objects to and from JSON. The standard Python libraries for encoding Python into JSON, such as the stdlib’s json, simplejson, and demjson, can only handle Python primitives that have a direct JSON equivalent (e.g. dicts, lists, strings, ints, etc.). jsonpickle builds on top of these libraries and allows more complex data structures to be serialized to JSON.

Emphasis mine.