Serializing output to JSON – ValueError: Circular reference detected

The function you pass as the default argument will only be called for objects that are not natively serializable by the json module. It must return a serializable object, or raise a TypeError.

Your version returns the same object you were passed if it’s not of the one type you’re fixing (dates). That is causing the circular reference error (which is misleading, since the circle is between one object and itself after being processed by date_handler).

You can start to fix this by changing date_handler to raise an exception in its else block. That will still probably fail, but you can probably then find out what object it is that is in your data structure causing the problem using code like this:

def date_handler(obj):
    if hasattr(obj, 'isoformat'):
        return obj.isoformat()
    else:
        raise TypeError(
            "Unserializable object {} of type {}".format(obj, type(obj))
        )