How to JSON serialize sets?

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You can create a custom encoder that returns a list when it encounters a set. Here’s an example:

>>> import json
>>> class SetEncoder(json.JSONEncoder):
...    def default(self, obj):
...       if isinstance(obj, set):
...          return list(obj)
...       return json.JSONEncoder.default(self, obj)
... 
>>> json.dumps(set([1,2,3,4,5]), cls=SetEncoder)
'[1, 2, 3, 4, 5]'

You can detect other types this way too. If you need to retain that the list was actually a set, you could use a custom encoding. Something like return {'type':'set', 'list':list(obj)} might work.

To illustrated nested types, consider serializing this:

>>> class Something(object):
...    pass
>>> json.dumps(set([1,2,3,4,5,Something()]), cls=SetEncoder)

This raises the following error:

TypeError: <__main__.Something object at 0x1691c50> is not JSON serializable

This indicates that the encoder will take the list result returned and recursively call the serializer on its children. To add a custom serializer for multiple types, you can do this:

>>> class SetEncoder(json.JSONEncoder):
...    def default(self, obj):
...       if isinstance(obj, set):
...          return list(obj)
...       if isinstance(obj, Something):
...          return 'CustomSomethingRepresentation'
...       return json.JSONEncoder.default(self, obj)
... 
>>> json.dumps(set([1,2,3,4,5,Something()]), cls=SetEncoder)
'[1, 2, 3, 4, 5, "CustomSomethingRepresentation"]'

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