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Bulletproof json.dump?

On 2020-07-06, Chris Angelico <rosuav at> wrote:
> On Mon, Jul 6, 2020 at 10:11 PM Jon Ribbens via Python-list
><python-list at> wrote:
>> While I agree entirely with your point, there is however perhaps room
>> for a bit more helpfulness from the json module. There is no sensible
>> reason I can think of that it refuses to serialize sets, for example.
> Sets don't exist in JSON. I think that's a sensible reason.

It is not. Tuples don't exist either, and yet they're supported.

>> Going a bit further and, for example, automatically calling isoformat()
>> on date/time/datetime objects would perhaps be a bit more controversial,
>> but would frequently be useful, and there's no obvious downside that
>> occurs to me.
> They wouldn't round-trip without some way of knowing which strings
> represent date/times.

The 'json' module already fails to provide round-trip functionality:

    >>> for data in ({True: 1}, {1: 2}, (1, 2)):
    ...     if json.loads(json.dumps(data)) != data:
    ...         print('oops', data, json.loads(json.dumps(data)))
    oops {True: 1} {'true': 1}
    oops {1: 2} {'1': 2}
    oops (1, 2) [1, 2]

> Maybe what people want is a pretty printer instead?

No. I want a JSON encoder to output JSON to be read by a JSON decoder.