Why does JSON serialization of datetime objects in Python not work out of the box for datetime objects?

Why does the JSON serialization not work for datetime objects? As I understand JSON serialization, the basic idea for any object can be called the __str__ built-in function and then URL encode the object that you get as a response. But in case of datetime, I get the following error

TypeError: datetime.datetime(2012, 5, 23, 18, 38, 23, 37566) is not JSON serializable

while there is a __str__, i.e., a way of stringifying the object already available , But it seems like a conscious decision to not do it , why would that be the case?

No, it doesn't work that way in the json module. The module provides you with a default encoder: json.JSONEncoder. You need to extend this to provide your implementation of default method to serialize objects. Something like this:

import json
import datetime
from time import mktime

class MyEncoder(json.JSONEncoder):

    def default(self, obj):
        if isinstance(obj, datetime.datetime):
            return int(mktime(obj.timetuple()))

        return json.JSONEncoder.default(self, obj)

print json.dumps(obj, cls=MyEncoder)

As others correctly pointed out, the reason is that the standard for JSON does not specify how date time can be represented.