How to change json encoding behaviour for serializable python object?

Question

It is easy to change the format of an object which is not JSON serializable eg datetime.datetime.

My requirement, for debugging purposes, is to alter the way some cu

Answer 1

If you define these to override __instancecheck__:

def strict_check(builtin):
    '''creates a new class from the builtin whose instance check
    method can be overridden to renounce particular types'''
    class BuiltIn(type):
        def __instancecheck__(self, other):
            print 'instance', self, type(other), other
            if type(other) in strict_check.blacklist:
                return False
            return builtin.__instancecheck__(other)
    # construct a class, whose instance check method is known.
    return BuiltIn('strict_%s' % builtin.__name__, (builtin,), dict())

# for safety, define it here.
strict_check.blacklist = ()

then patch json.encoder like this to override _make_iterencode.func_defaults:

# modify json encoder to use some new list/dict attr.
import json.encoder
# save old stuff, never know when you need it.
old_defaults = json.encoder._make_iterencode.func_defaults
old_encoder = json.encoder.c_make_encoder
encoder_defaults = list(json.encoder._make_iterencode.func_defaults)
for index, default in enumerate(encoder_defaults):
    if default in (list, dict):
        encoder_defaults[index] = strict_check(default)

# change the defaults for _make_iterencode.
json.encoder._make_iterencode.func_defaults = tuple(encoder_defaults)
# disable C extension.
json.encoder.c_make_encoder = None

... your example would almost work verbatim:

import datetime
import json

def json_debug_handler(obj):
    print("object received:")
    print type(obj)
    print("\n\n")
    if  isinstance(obj, datetime.datetime):
        return obj.isoformat()
    elif isinstance(obj,mDict):
        # degrade obj to more primitive dict()
        # to avoid cycles in the encoding.
        return {'orig': dict(obj) , 'attrs': vars(obj)}
    elif isinstance(obj,mList):
        # degrade obj to more primitive list()
        # to avoid cycles in the encoding.
        return {'orig': list(obj), 'attrs': vars(obj)}
    else:
        return None


class mDict(dict):
    pass


class mList(list):
    pass

# set the stuff we want to process differently.
strict_check.blacklist = (mDict, mList)

def test_debug_json():
    global test_json
    games = mList(['mario','contra','tetris'])
    games.src = 'console'
    scores = mDict({'dp':10,'pk':45})
    scores.processed = "unprocessed"
    test_json = { 'games' : games , 'scores' : scores , 'date': datetime.datetime.now() }
    print(json.dumps(test_json,default=json_debug_handler))

if __name__ == '__main__':
    test_debug_json()

The things I needed to change were to make sure there were no cycles:

    elif isinstance(obj,mDict):
        # degrade obj to more primitive dict()
        # to avoid cycles in the encoding.
        return {'orig': dict(obj) , 'attrs': vars(obj)}
    elif isinstance(obj,mList):
        # degrade obj to more primitive list()
        # to avoid cycles in the encoding.
        return {'orig': list(obj), 'attrs': vars(obj)}

and add this somewhere before test_debug_json:

# set the stuff we want to process differently.
strict_check.blacklist = (mDict, mList)

here is my console output:

>>> test_debug_json()
instance <class '__main__.strict_list'> <type 'dict'> {'date': datetime.datetime(2013, 7, 17, 12, 4, 40, 950637), 'games': ['mario', 'contra', 'tetris'], 'scores': {'pk': 45, 'dp': 10}}
instance <class '__main__.strict_dict'> <type 'dict'> {'date': datetime.datetime(2013, 7, 17, 12, 4, 40, 950637), 'games': ['mario', 'contra', 'tetris'], 'scores': {'pk': 45, 'dp': 10}}
instance <class '__main__.strict_list'> <type 'datetime.datetime'> 2013-07-17 12:04:40.950637
instance <class '__main__.strict_dict'> <type 'datetime.datetime'> 2013-07-17 12:04:40.950637
instance <class '__main__.strict_list'> <type 'datetime.datetime'> 2013-07-17 12:04:40.950637
instance <class '__main__.strict_dict'> <type 'datetime.datetime'> 2013-07-17 12:04:40.950637
object received:
<type 'datetime.datetime'>



instance <class '__main__.strict_list'> <class '__main__.mList'> ['mario', 'contra', 'tetris']
instance <class '__main__.strict_dict'> <class '__main__.mList'> ['mario', 'contra', 'tetris']
instance <class '__main__.strict_list'> <class '__main__.mList'> ['mario', 'contra', 'tetris']
instance <class '__main__.strict_dict'> <class '__main__.mList'> ['mario', 'contra', 'tetris']
object received:
<class '__main__.mList'>



instance <class '__main__.strict_list'> <type 'dict'> {'attrs': {'src': 'console'}, 'orig': ['mario', 'contra', 'tetris']}
instance <class '__main__.strict_dict'> <type 'dict'> {'attrs': {'src': 'console'}, 'orig': ['mario', 'contra', 'tetris']}
instance <class '__main__.strict_list'> <type 'dict'> {'src': 'console'}
instance <class '__main__.strict_dict'> <type 'dict'> {'src': 'console'}
instance <class '__main__.strict_list'> <type 'list'> ['mario', 'contra', 'tetris']
instance <class '__main__.strict_list'> <class '__main__.mDict'> {'pk': 45, 'dp': 10}
instance <class '__main__.strict_dict'> <class '__main__.mDict'> {'pk': 45, 'dp': 10}
instance <class '__main__.strict_list'> <class '__main__.mDict'> {'pk': 45, 'dp': 10}
instance <class '__main__.strict_dict'> <class '__main__.mDict'> {'pk': 45, 'dp': 10}
object received:
<class '__main__.mDict'>



instance <class '__main__.strict_list'> <type 'dict'> {'attrs': {'processed': 'unprocessed'}, 'orig': {'pk': 45, 'dp': 10}}
instance <class '__main__.strict_dict'> <type 'dict'> {'attrs': {'processed': 'unprocessed'}, 'orig': {'pk': 45, 'dp': 10}}
instance <class '__main__.strict_list'> <type 'dict'> {'processed': 'unprocessed'}
instance <class '__main__.strict_dict'> <type 'dict'> {'processed': 'unprocessed'}
instance <class '__main__.strict_list'> <type 'dict'> {'pk': 45, 'dp': 10}
instance <class '__main__.strict_dict'> <type 'dict'> {'pk': 45, 'dp': 10}
{"date": "2013-07-17T12:04:40.950637", "games": {"attrs": {"src": "console"}, "orig": ["mario", "contra", "tetris"]}, "scores": {"attrs": {"processed": "unprocessed"}, "orig": {"pk": 45, "dp": 10}}}