Published: 03 December 2020, 18:00
json.dumps
can serialize every built-in type which has a corresponding JSON type (int
as number
, None
as null
, list
as array
etc) but fails for every other type. Probably, the most often case when you will face it is when trying to serialize a datetime object:
import json
from datetime import datetime
json.dumps([123, 'hello'])
# '[123, "hello"]'
json.dumps(datetime.now())
# TypeError: Object of type 'datetime' is not JSON serializable
The fastest way to fix it is to provide a custom default serializer:
json.dumps(datetime.now(), default=str)
# '"2020-12-03 18:00:10.592496"'
However, that means that every unknown object will be serialized into a string which can lead to unexpected result:
class C: pass
json.dumps(C(), default=str)
'"<__main__.C object at 0x7f330ec801d0>"'
So, if you want to serialize only datetime
and nothing else, it’s better to define a custom encoder:
class DateTimeEncoder(json.JSONEncoder):
def default(self, obj) -> str:
if isinstance(obj, datetime):
return obj.isoformat()
return super().default(obj)
json.dumps(datetime.now(), cls=DateTimeEncoder)
'"2020-12-03T18:01:19.609648"'
json.dumps(C(), cls=DateTimeEncoder)
# TypeError: Object of type 'C' is not JSON serializable