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?

Asked By: orokusaki

||

Source

Answer 1

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.

Answered By: Vinay Sajip

Answer 2

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'

Answered By: Ted Dunning

Answer 3

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.

Answered By: hobs

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

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