Django: "TypeError: [] is not JSON serializable" Why?

Question

How can this be that this error was raised? I entered this:

def json(self):
    return json.dumps(
        {
            'items': self.items
        }
    )

and got that error (because self.items was an empty queryset (Django)

but then,

def json(self):
    return json.dumps(
        {
            'items': []  # Pass in empty list to prove that the error was idiotic.
        }
    )

worked fine (which at least proves that the error message is worthless)

Is this because the queryset defines repr() and returns '[]' as a string when it's empty or something ridiculous like that?

Why are you defining a function with the same name as a module you are using? — John La Rooy
– John La Rooy, Commented Feb 11, 2010 at 8:06
@gnibbler It's not a function. It's a method and I'm using it as a @property (didn't include that in my example). — orokusaki
– orokusaki, Commented Feb 11, 2010 at 8:10
I love looking back at how smug I was, ...the error was idiotic. — orokusaki
– orokusaki, Commented Nov 15, 2013 at 15:44

Vinay Sajip · Accepted Answer · 2010-02-11 20:03:44Z

60

Querysets are not serializable out-of-the-box. If you try list(self.items) instead of just self.items, that should work as long as the items themselves are JSON-serializable.

Update: It will raise an exception even if it isn't empty. I don't think it'll be accepted as a Django bug, though of course you can try; the simplest answer is to force evaluation using list(qs), as I've already said.

edited Feb 11, 2010 at 20:03

answered Feb 11, 2010 at 10:05

Vinay Sajip

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3 Comments

cethegeek Over a year ago

+1. This is the answer. You are using list(queryset) to force the queryset to be evaluated right away. The underlying data, in the form of a list, is serializable; the queryset itself is not. docs.djangoproject.com/en/dev/ref/models/querysets/#id1

orokusaki Over a year ago

Good answer. Thanks. BTW, This seems like a bug. I'm going to start a ticket for it. Django's serializers are meant for serializing Django querysets. If it raises an exception when it's empty, that's a bug.

hobs Over a year ago

@orokusaki, this doesn't feel like a bug to me. Sometimes you want objects. Sometimes you want strings. Django lets you chose. Here's a more pythonic alternative that works for me, for all Django fields: [str(obj) for obj in Model.objects.values()] see below

Ted Dunning · Accepted Answer · 2012-03-17 01:49:26Z

This is very frustrating. Django's serialization complains about everything that isn't a query set and json.dumps complains about objects from Django's ORM support.

>>> from cluster.models import Account
>>> import json
>>> json.dumps(Account.objects.all()[0])
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py", line 201, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py", line 264, in iterencode
    return _iterencode(o, 0)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py", line 178, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: <Account: 9de5-2653-000d-81a3 => [email protected]> is not JSON serializable

Versus

>>> serializers.serialize("json", [clusters])
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/core/serializers/__init__.py", line 91, in serialize
    s.serialize(queryset, **options)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/core/serializers/base.py", line 41, in serialize
    for field in obj._meta.local_fields:
AttributeError: 'QuerySet' object has no attribute '_meta'

hobs · Accepted Answer · 2013-02-01 19:52:03Z

4

As Vinay pointed out, even if you cast to a list, serialization often still fails. For me, serialization fails on DateTimeField elements (datetime.datetime objects), even if I ask for a ValuesQuerySet (list-like) with .values(). The solution for me was a simple comprehension.

json.dumps([str(obj) for obj in Model.objects.values()]);

In your case, that would be

return json.dumps({k: str(v) for k, v in self.__dict__.items()})

The magic of str saves the day. The repr built-in may also be helpful if you need object type information in your serialization.

edited Feb 1, 2013 at 19:52

answered Feb 1, 2013 at 19:10

hobs

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