Dinesh Dinesh - 2 months ago 12
Python Question

Python to JSON Serialization fails on Decimal

I have a python object which includes some decimals. This is causing the json.dumps() to break.

I got the following solution from SO (e.g. Python JSON serialize a Decimal object) but
the recoomended solution still does not work. Python website - has the exact same answer.

Any suggestions how to make this work?

Thanks. Below is my code. It looks like the dumps() doesn't even go into the specialized encoder.

clayton@mserver:~/python> cat test1.py
import json, decimal

class DecimalEncoder(json.JSONEncoder):
def _iterencode(self, o, markers=None):
print "here we go o is a == ", type(o)
if isinstance(o, decimal.Decimal):
print "woohoo! got a decimal"
return (str(o) for o in [o])
return super(DecimalEncoder, self)._iterencode(o, markers)

z = json.dumps( {'x': decimal.Decimal('5.5')}, cls=DecimalEncoder )
print z
clayton@mserver:~/python> python test1.py
Traceback (most recent call last):
File "test1.py", line 11, in <module>
z = json.dumps( {'x': decimal.Decimal('5.5')}, cls=DecimalEncoder )
File "/home/clayton/python/Python-2.7.3/lib/python2.7/json/__init__.py", line 238, in dumps
**kw).encode(obj)
File "/home/clayton/python/Python-2.7.3/lib/python2.7/json/encoder.py", line 201, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/home/clayton/python/Python-2.7.3/lib/python2.7/json/encoder.py", line 264, in iterencode
return _iterencode(o, 0)
File "/home/clayton/python/Python-2.7.3/lib/python2.7/json/encoder.py", line 178, in default
raise TypeError(repr(o) + " is not JSON serializable")
TypeError: Decimal('5.5') is not JSON serializable
clayton@mserver:~/python>

Answer

It is not (no longer) recommended you create a subclass; the json.dump() and json.dumps() functions take a default function:

def decimal_default(obj):
    if isinstance(obj, decimal.Decimal):
        return float(obj)
    raise TypeError

json.dumps({'x': decimal.Decimal('5.5')}, default=decimal_default)

Demo:

>>> def decimal_default(obj):
...     if isinstance(obj, decimal.Decimal):
...         return float(obj)
...     raise TypeError
... 
>>> json.dumps({'x': decimal.Decimal('5.5')}, default=decimal_default)
'{"x": 5.5}'

The code you found only worked on Python 2.6 and overrides a private method that is no longer called in later versions.