json encoder unable to handle decimal

[merwok]

BPO	16535
Nosy	@rhettinger, @jcea, @etrepum, @mdickinson, @pitrou, @tiran, @ezio-melotti, @merwok, @serhiy-storchaka, @lesinigo
Files	json_decimal.patch: Patch file generated via "hg diff"

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2012-11-22.22:45:38.401>
labels = ['type-feature', 'library', '3.10']
title = 'json encoder unable to handle decimal'
updated_at = <Date 2022-03-03.10:12:29.632>
user = 'https://github.com/merwok'

bugs.python.org fields:

activity = <Date 2022-03-03.10:12:29.632>
actor = 'luca.lesinigo'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2012-11-22.22:45:38.401>
creator = 'eric.araujo'
dependencies = []
files = ['30836']
hgrepos = []
issue_num = 16535
keywords = ['patch']
message_count = 16.0
messages = ['176135', '176136', '176158', '176161', '176398', '179869', '192520', '213381', '214733', '214737', '224032', '224096', '274026', '289010', '349250', '383559']
nosy_count = 16.0
nosy_names = ['rhettinger', 'jcea', 'bob.ippolito', 'mark.dickinson', 'pitrou', 'christian.heimes', 'ezio.melotti', 'eric.araujo', 'Arfrever', 'zzzeek', 'cvrebert', 'serhiy.storchaka', 'ralhei', 'mjensen', 'risa2000', 'luca.lesinigo']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue16535'
versions = ['Python 3.10']

[merwok]

In 2.7 and other versions, the json module has incomplete support for decimals:

>>> json.loads('0.2', parse_float=Decimal)
Decimal('0.2')
>>> json.dumps(json.loads('0.2', parse_float=Decimal))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python2.7/json/__init__.py", line 231, in dumps
 return _default_encoder.encode(obj)
 File "/usr/lib/python2.7/json/encoder.py", line 201, in encode
 chunks = self.iterencode(o, _one_shot=True)
 File "/usr/lib/python2.7/json/encoder.py", line 264, in iterencode
 return _iterencode(o, 0)
 File "/usr/lib/python2.7/json/encoder.py", line 178, in default
 raise TypeError(repr(o) + " is not JSON serializable")
TypeError: Decimal('0.2') is not JSON serializable

simplejson encodes decimals out of the box, but json can’t round-trip.

[merwok]

See lengthy discussion that lead to inclusion in simplejson here: http://code.google.com/p/simplejson/issues/detail?id=34

[serhiy-storchaka]

The json module already has too many options. No need for yet one such specialized.

>>> class number_str(float):
... def __init__(self, o):
... self.o = o
... def __repr__(self):
... return str(self.o)
... 
>>> def decimal_serializer(o):
... if isinstance(o, decimal.Decimal):
... return number_str(o)
... raise TypeError(repr(o) + " is not JSON serializable")
... 
>>> print(json.dumps([decimal.Decimal('0.20000000000000001')], default=decimal_serializer))
[0.20000000000000001]

You can extend this to support complex numbers, fractions, date and time, and many other custom types. Have specialized options for this would be cumbersome.

[mdickinson]

Judging by the discussion that Éric points to, and by the various stackoverflow questions on the topic ([1], [2]), this is a common enough need that I think it would make sense to have some support for it in the std. lib.

There's a sense in which Decimal is the 'right' type for json, and we shouldn't make it harder for people to do the right thing with respect to (e.g.) financial data in databases.

[1] http://stackoverflow.com/questions/4019856/decimal-to-json
[2] http://stackoverflow.com/questions/1960516/python-json-serialize-a-decimal-object

[merwok]

Thanks for the workaround Serhiy, I stole that and ran with it :)

For 3.4 I still think something built-in would be best.

[pitrou]

Decimal numbers should simply be serializable by default. It doesn't make sense to add a specialized option.

[ralhei]

This patch was implemented on Europython 2013 sprint. It's my first addition to Python core ever so please bear with me if it's not perfect.

Decimal support is implemented both in the C and Python JSON code.

There is one peculiarity to mention about the Decimal addition in function _json.c:encoder_listencode_obj() of my patch:
The addition of

 else if (PyObject_IsInstance(obj, (PyObject*)PyDecimalType)) {
 PyObject *encoded = encoder_encode_decimal(s, obj);
 if (encoded == NULL)
 return -1;
 return _steal_accumulate(acc, encoded);
 }

was has to be located AFTER lists and dicts are handled in the JSON encoder, otherwise the unittest "test_highly_nested_objects_encoding()" from test_recursion.py fails with a nasty, unrecoverable Python exception.
My guess is that this is due additional stack allocation when the stack space is almost used up by the deeply nested recursion code.

[merwok]

Patch looks good and contains tests for the C and Python code.

Documentation is missing (a note to tell that json.dump converts decimal.Decimal instances to JSON numbers, a versionchanged directive, maybe a link to the doc that explains parse_float=decimal.Decimal).

[pitrou]

The patch isn't really ok, IMO. It forcibly imports the decimal module and then looks up the type there. The decimal module is a rather large one and it shouldn't get imported if it doesn't get used.

I think it would be better to rely on the __float__ special method, which would also automatically accept other numberish types such as Fraction.

[skrah]

I think we should really apply bpo-19232. At least that would take care
of the import issue.

[tiran]

I'm EuroPython 2014 in Berlin. Ralph has approached me and asked me about progress on the progress of this patch. I'm reluctant to implement a special case for decimals for two reasons:

1. JSON just support floats and decimals are IMHO incompatible with floats. The conversion of decial to JSON floats is a loosely operation.

2. Rather than having a special case I would rather go with a general implementation that uses an ABC to JSON dump some float-like objects.