json.dump not calling default or cls

TLDR: .default is only called for dict values, not keys.

To elaborate on Abhishek Kumars answer; after reviewing the json.JSONEncoder source, .default is only ever called on dict values that are not serializable, never keys. Keys must be one of str, int, float, bool or None.

To use datetime objects as keys, .iterencode must be overwritten to use recursive pre-processor method to convert datetime to str:

test = {        "key_1": "Value_1",        "key_2": 10,        "key_3": ["list_" + str(i) for i in range(5)],        "key_4": {"nestkey_" + str(i) : "nestvalue_" + str(i) for i in range(5) },        "key_5": datetime.datetime(1970, 1, 1, 8, 0, 0),        datetime.datetime(1970, 1, 1, 8, 0, 0): "datetime_key"}class DateTimeEncoder(json.JSONEncoder):    def _preprocess_date(self, obj):        if isinstance(obj, (datetime.date, datetime.datetime, datetime.timedelta)):            return str(obj)        elif isinstance(obj, dict):            return {self._preprocess_date(k): self._preprocess_date(v) for k,v in obj.items()}        elif isinstance(obj, list):            return [self._preprocess_date(i) for i in obj]        return obj    def default(self, obj):        if isinstance(obj, (datetime.date, datetime.datetime, datetime.timedelta)):            return str(obj)        return super().default(obj)    def iterencode(self, obj):        return super().iterencode(self._preprocess_date(obj))with open('output.json', 'w') as fo:    json.dump(test, fo, cls=DateTimeEncoder)

This is obviously an expensive operation for larger dictionaries so some caution should be exercised. Also, would be nice for json.JSONEncoder to be updated to use default on keys as well as values - issue created at : https://bugs.python.org/issue41569

you can try this :

class DatesToStrings(json.JSONEncoder):    def _encode(self, obj):        if isinstance(obj, dict):            def transform_date(o):                return self._encode(o.isoformat() if isinstance(o, datetime) else o)            return {transform_date(k): transform_date(v) for k, v in obj.items()}        else:            return obj    def encode(self, obj):        return super(DatesToStrings, self).encode(self._encode(obj))

with open("output.json", "w") as fo:    json.dump( json.dumps( test, cls=DatesToStrings), fo, cls=DatesToStrings)