set object is not JSON serializable

Question

When I try to run the following code:

import json

d = {'testing': {1, 2, 3}}
json_string = json.dumps(d)

I get the following exception:

Traceback (most recent call last):
  File "json_test.py", line 4, in <module>
    json_string = json.dumps(d)
  File "/usr/lib/python2.7/json/__init__.py", line 243, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: set([1, 2, 3]) is not JSON serializable

What can I do to successfully use json.dumps with objects containing sets?

score 83 · Accepted Answer · answered Mar 09 '14 at 10:26

83

Turn sets into lists before serializing, or use a custom default handler to do so:

def set_default(obj):
    if isinstance(obj, set):
        return list(obj)
    raise TypeError

result = json.dumps(yourdata, default=set_default)

answered Mar 09 '14 at 10:26

Martijn Pieters

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Kei Minagawa · Answer 2 · 2014-03-10T13:16:54.277

2

You can't fix it.

This error means just "json.dumps doesn't support data type "set".You should know JSON comes from javascript. And there is no data type like Python's "set" in javascript. So Python can't treat 'set' using JSON.

So you need another approach like @Martijn Pieters mentioned.

UPDATE

I forgot to say this.

If you want to dump "set" or any other python object that is not supported JSON, you can use pickle or cPickle module. If you use the "dump.txt" only from Python, this may be helpful.

import cPickle

d = {'testing': {1, 2, 3}}

#dump
with open("pickledump.txt", "w") as fp:
    cPickle.dump(d, fp)

#load
with open("pickledump.txt", "r") as fp:
    x = cPickle.load(fp)

edited Mar 10 '14 at 13:16

answered Mar 09 '14 at 10:55

Kei Minagawa

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Note my downvote, but I'd hazard a guess the downvote was based on your original post which could be read as: "you can't do that" (which the OP obviously knows as that's what they're asking about) followed by "see Martijn's answer". – Jon Clements Mar 10 '14 at 18:02
2

Yes...you're right. What I should say is "You need another approach, because you can't store Python's 'set' as json format." – Kei Minagawa Mar 10 '14 at 22:50

set object is not JSON serializable

2 Answers2

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