Python serialize objects list to JSON [duplicate]

Question

I am trying to serialize to JSON the __dict__ of an object, which is working fine, until I append objects to one of the instance attribute of my first object:

from json import dumps

class A(object):
    def __init__(self):
        self.b_list = []

class B(object):
    def __init__(self):
        self.x = 'X'
        self.y = 'Y'
    def __repr__(self):
        return dumps(self.__dict__)

a = A()

print dumps(a.__dict__)  # works fine

a.b_list.append(B())

print dumps(a.__dict__)

When calling for the second time dumps, I got the following TypeError:

TypeError: {"y": "Y", "x": "X"} is not JSON serializable

I don't understand why I keep getting this error while I can't see why this is not serializable to JSON.

Martijn Pieters · Accepted Answer · 2015-07-23 11:04:07Z

7

That's because instances of B are not a simple type. Because you gave B a __repr__ method, the instance is printed as it's JSON representation, but it is not itself a supported JSON type.

Remove the __repr__ method and the traceback is much less confusing:

>>> class A(object):
...     def __init__(self):
...         self.b_list = []
... 
>>> class B(object):
...     def __init__(self):
...         self.x = 'X'
...         self.y = 'Y'
... 
>>> a = A()
>>> a.b_list.append(B())
>>> 
>>> print dumps(a.__dict__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/__init__.py", line 243, in dumps
    return _default_encoder.encode(obj)
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/Users/mj/Development/Library/buildout.python/parts/opt/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: <__main__.B object at 0x10a753e10> is not JSON serializable

Use the default keyword argument to encode custom objects:

def encode_b(obj):
    if isinstance(obj, B):
        return obj.__dict__
    return obj

json.dumps(a, default=encode_b)

Demo:

>>> def encode_b(obj):
...     if isinstance(obj, B):
...         return obj.__dict__
...     return obj
... 
>>> dumps(a.__dict__, default=encode_b)
'{"b_list": [{"y": "Y", "x": "X"}]}'

edited Jul 23, 2015 at 11:04

answered Nov 19, 2013 at 18:08

Martijn Pieters

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6 Comments

evuez Over a year ago

thanks for the explanation and the demo! I thought json.dumps would use __repr__ to serialize the objects, but without __repr__ it is indeed much more clear.

Tjorriemorrie Over a year ago

That doesn't make sense to me: if B's repr returns a string, surely that's a simple type? This does not clear up the understanding of json encoding repr :(

Martijn Pieters Over a year ago

@Tjorriemorrie: JSON doesn't use __repr__. The OP tried to make instances of B serialisable to JSON by giving them a __repr__ method, but that method isn't used here at all.

Tjorriemorrie Over a year ago

Is there a way to add a decorator to an object that will then handle the encode logic/process automatically when it gets encoded instead of extending the JsonEncoder? (I assume there is no such attribute that json looks for on an object?)

Martijn Pieters Over a year ago

@Tjorriemorrie: You could use a decorator but you'll still have to extend JSONEncoder, because indeed there is no attribute that the encoder looks for. The Pyramid project extended their encoder to look for __json__ methods, but again, it requires a custom encoder.

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