It seems that to achieve the behavior you want, with the given restrictions, you'll have to delve into the JSONEncoder
class a little. Below I've written out a custom JSONEncoder
that overrides the iterencode
method to pass a custom isinstance
method to _make_iterencode
. It isn't the cleanest thing in the world, but seems to be the best given the options and it keeps customization to a minimum.
# customencoder.py
from json.encoder import (_make_iterencode, JSONEncoder,
encode_basestring_ascii, FLOAT_REPR, INFINITY,
c_make_encoder, encode_basestring)
class CustomObjectEncoder(JSONEncoder):
def iterencode(self, o, _one_shot=False):
"""
Most of the original method has been left untouched.
_one_shot is forced to False to prevent c_make_encoder from
being used. c_make_encoder is a funcion defined in C, so it's easier
to avoid using it than overriding/redefining it.
The keyword argument isinstance for _make_iterencode has been set
to self.isinstance. This allows for a custom isinstance function
to be defined, which can be used to defer the serialization of custom
objects to the default method.
"""
# Force the use of _make_iterencode instead of c_make_encoder
_one_shot = False
if self.check_circular:
markers = {}
else:
markers = None
if self.ensure_ascii:
_encoder = encode_basestring_ascii
else:
_encoder = encode_basestring
if self.encoding != 'utf-8':
def _encoder(o, _orig_encoder=_encoder, _encoding=self.encoding):
if isinstance(o, str):
o = o.decode(_encoding)
return _orig_encoder(o)
def floatstr(o, allow_nan=self.allow_nan,
_repr=FLOAT_REPR, _inf=INFINITY, _neginf=-INFINITY):
if o != o:
text = 'NaN'
elif o == _inf:
text = 'Infinity'
elif o == _neginf:
text = '-Infinity'
else:
return _repr(o)
if not allow_nan:
raise ValueError(
"Out of range float values are not JSON compliant: " +
repr(o))
return text
# Instead of forcing _one_shot to False, you can also just
# remove the first part of this conditional statement and only
# call _make_iterencode
if (_one_shot and c_make_encoder is not None
and self.indent is None and not self.sort_keys):
_iterencode = c_make_encoder(
markers, self.default, _encoder, self.indent,
self.key_separator, self.item_separator, self.sort_keys,
self.skipkeys, self.allow_nan)
else:
_iterencode = _make_iterencode(
markers, self.default, _encoder, self.indent, floatstr,
self.key_separator, self.item_separator, self.sort_keys,
self.skipkeys, _one_shot, isinstance=self.isinstance)
return _iterencode(o, 0)
You can now subclass the CustomObjectEncoder
so it correctly serializes your custom objects. The CustomObjectEncoder
can also do cool stuff like handle nested objects.
# test.py
import json
import datetime
from customencoder import CustomObjectEncoder
class MyEncoder(CustomObjectEncoder):
def isinstance(self, obj, cls):
if isinstance(obj, (mList, mDict)):
return False
return isinstance(obj, cls)
def default(self, obj):
"""
Defines custom serialization.
To avoid circular references, any object that will always fail
self.isinstance must be converted to something that is
deserializable here.
"""
if isinstance(obj, datetime.datetime):
return obj.isoformat()
elif isinstance(obj, mDict):
return {"orig": dict(obj), "attrs": vars(obj)}
elif isinstance(obj, mList):
return {"orig": list(obj), "attrs": vars(obj)}
else:
return None
class mList(list):
pass
class mDict(dict):
pass
def main():
zelda = mList(['zelda'])
zelda.src = "oldschool"
games = mList(['mario', 'contra', 'tetris', zelda])
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, cls=MyEncoder))
if __name__ == '__main__':
main()