Bulletproof json.dump?

[Jon Ribbens]

On 2020-07-06, Frank Millman <frank at chagford.com> wrote:
> On 2020-07-06 3:08 PM, Jon Ribbens via Python-list wrote:
>> On 2020-07-06, Frank Millman <frank at chagford.com> wrote:
>>> On 2020-07-06 2:06 PM, Jon Ribbens via Python-list 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.
>>>> 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.
>>>
>>> I may be missing something, but that would cause a downside for me.
>>>
>>> I store Python lists and dicts in a database by calling dumps() when
>>> saving them to the database and loads() when retrieving them.
>>>
>>> If a date was 'dumped' using isoformat(), then on retrieval I would not
>>> know whether it was originally a string, which must remain as is, or was
>>> originally a date object, which must be converted back to a date object.
>>>
>>> There is no perfect answer, but my solution works fairly well. When
>>> dumping, I use 'default=repr'. This means that dates get dumped as
>>> 'datetime.date(2020, 7, 6)'. I look for that pattern on retrieval to
>>> detect that it is actually a date object.
>> 
>> There is no difference whatsoever between matching on the repr output
>> you show above and matching on ISO-8601 datetimes, except that at least
>> ISO-8601 is an actual standard. So no, you haven't found a downside.
>
> I don't understand. As you say, ISO-8601 is a standard, so the original 
> object could well have been a string in that format. So how do you 
> distinguish between an object that started out as a string, and an 
> object that started out as a date/datetime object?

With your method, how do you distinguish between an object that started
out as a string, and an object that started out as a date/datetime
object? The answer with both my method and your method is that you
cannot - and therefore my method is not a "downside" compared to yours.
Not to mention, I am not suggesting that your method should be
disallowed if you want to continue using it - I am suggesting that
your code could be simplified and your job made easier by my suggested
improvement.