Django DecimalField is rendered as string in json

[yprez]

This was introduced in 2.1.16

[tomchristie]

What makes you think this is introduced in 2.1.16?
As far as I'm aware Decimals have always (intentionally) rendered as strings in json.

JSON has no decimal representation, so it's not obvious what sensible behavior is. See #508.

If you treat them as floats you get anomalies due to binary floating point not being able to precisely represent decimal numbers.

eg. try this in a python console...

>>> 0.1
0.10000000000000001

[tomchristie]

That's not to say that we couldn't consider instead rendering them using a correct decimal notation in json, but it's not obvious if that's a sensible thing to do given that clients will then decode that into binary floating point.

[yprez]

Yes... weird
After looking at the encoders file, it doesn't make much sense.
However, I'm absolutely sure that decimals were rendered as floats in 2.1.15

Not sure what fixed it, but it should probably be in the changelog in case someone relies on it...

[yprez]

with 2.1.15:

>>> from decimal import Decimal
>>> from rest_framework import renderers
>>> 
>>> obj = {'foo': ['bar', 'baz'], 'float': 1.1, 'decimal': Decimal(1.1)}
>>> 
>>> renderers.JSONRenderer().render(obj, 'application/json')
'{"float": 1.1, "decimal": 1.100000000000000088817841970012523233890533447265625, "foo": ["bar", "baz"]}'`

with 2.1.16:

>>> renderers.JSONRenderer().render(obj, 'application/json')
'{"float": 1.1, "decimal": "1.100000000000000088817841970012523233890533447265625", "foo": ["bar", "baz"]}'

[tomchristie]

Urg? Don't know how the behavior's changed, but in either case 2.1.15 is broken, and 2.1.16 is even more broken. :-/

(If it's going to be a string repr it should at least be a decimal string repr)

Note that if you want to put together a failing test case for DecimalFields, this module is intended for basic field tests: https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/tests/fields.py

[yprez]

My mistake... I should've created decimals from strings not floats...

>>> str(Decimal(1.1))
'1.100000000000000088817841970012523233890533447265625'

The types are still the same though...

>>> obj = {'foo': ['bar', 'baz'], 'float': 1.1, 'decimal': Decimal('1.1')}
>>> 
>>> renderers.JSONRenderer().render(obj, 'application/json')
'{"float": 1.1, "decimal": 1.1, "foo": ["bar", "baz"]}'

[yprez]

I can't replicate it in the DRF env for some reason... this is getting really weird.

[yprez]

Ok... found it... a061e3d
It's the switch from simplejson to the builtin that caused this for me...

[yprez]

@tomchristie, So what should we do with the decimals? Leave them as strings?

Before 2.1.16, users who had simplejson installed had them rendered as floats, and without it as strings...
Maybe leave it as is and add a note in the changelog?

[minddust]

+1 for accuracy and strings

[yprez]

Awesome, thanks.

[StErMi]

Django==1.9.4
djangorestframework==3.3.2

Is this bug still opened? Because I was just using DecimalField and in my JSON it's rappresented as a string.

UPDATE: my bad, din't see the coerce_to_string parameter. Sorry guys!

[SalahAdDin]

Which parameter?

[ChristopherBrix]

@SalahAdDin: It's a parameter for the DecimalField

[ghost]

The coerce_to_string param is available to 2.3.14? i getting the error got an unexpected keyword argument 'decimal_places' :/ Any tips on how to make it work on 2.3?