Assuming you can not change the original modules source...

A way to do that is without generic functions is below:

from decimal import Decimal
from jsonify import jsonify

jsonify_orig = jsonify
def jasonify_mods(obj):
    if isinstance(obj,Decimal):
        ...
    else:
        return jsonify_orig

jsonify = jasonify_mods


Why are generic functions better than this technique?