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?

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.

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'