How to use custom Python JSON serializers and deserializers to automatically roundtrip complex types.

gistfile1.py

import json, datetime

class RoundTripEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime.datetime):
            return "$dt$" + str(obj.isoformat())
        return super().default(obj)

data = {
    "name": "Silent Bob",
    "dt": datetime.datetime(2013, 11, 11, 10, 40, 32)
}

s = json.dumps(data, cls=RoundTripEncoder, indent=2)
print(s)

class RoundTripDecoder(json.JSONDecoder):
    def __init__(self, *args, **kwargs):
        json.JSONDecoder.__init__(self, object_hook=self.object_hook, *args, **kwargs)

    def object_hook(self, obj):
        for k, v in obj.items():
            if isinstance(v, str) and v[0:4] == "$dt$":
                obj[k] = datetime.datetime.fromisoformat(v[4:])
        return obj

print (json.loads(s, cls=RoundTripDecoder))

Output:

{
  "name": "Silent Bob",
  "dt": "$dt$2013-11-11T10:40:32"
}
{'name': 'Silent Bob', 'dt': datetime.datetime(2013, 11, 11, 10, 40, 32)}

The trick is to use $dt$ to at the beginning of datetime like $6$ for password hash. If it might be possible this string is legally in the json serialized string: you can rewrite line 23 with a try catch:

-            if isinstance(v, str) and v[0:4] == "$dt$":
+            try:
+                if isinstance(v, str):
-                obj[k] = datetime.datetime.fromisoformat(v[4:])
+                    obj[k] = datetime.datetime.fromisoformat(v[4:])
+            except ValueError: pass