Python 3.5 JSON 序列化一个十进制 object

Question

I need to encode into json a decimal value: 999999.99990000005 while not losing precision and not changing representation to string. Expecting { "prc": 999999.99990000005 }

From [this post][1] i have this code.

import json
import decimal

class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            return str(o)
        return super(DecimalEncoder, self).default(o)

y = { 'prc' : decimal.Decimal('999999.99990000005')}

but it produces a string

json.dumps(y, cls=DecimalEncoder)

'{"cPrc": "999999.99990000005"}'

replacing str(o) with float(o) in isinstance truncates the number. Any way to get the non-string result? PS I cannot use any external modules like simplejson.

EDIT: If I keep the value as a string, following produces a string too.

>>> x = json.loads("""{ "cPrc" : "999999.99990000005" }""", parse_float=decimal.Decimal)
>>> x
{'cPrc': '999999.99990000005'}

Answer 1

It is not the most pretty but if you insist on using json we can create a custom decoder and have our encoder specify the type when handling decimal data.

class DecimalEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, decimal.Decimal):
            return {
                "_type": "decimal",
                "value": str(obj)
            }
        return super(DecimalEncoder, self).default(obj)

The code above adds the decimal type as a flag for our decoder as well as encoding the decimal as a string to keep precision.

class DecimalDecoder(json.JSONDecoder):
    def __init__(self, *args, **kwargs):
        json.JSONDecoder.__init__(self, object_hook=self.object_hook, *args, **kwargs)

    def object_hook(self, obj):
        if '_type' not in obj:
            return obj
        type = obj['_type']
        if type == 'decimal':
            return decimal.Decimal(obj['value'])
        return obj

The decoder checks for our decimal type flag and if so uses the decimal constructor. For all other instances it uses default decoding

input = { 'prc' : decimal.Decimal('999999.99990000005')}
encoded = json.dumps(input, cls=DecimalEncoder)
decoded = json.loads(encoded, cls=DecimalDecoder)

The final result should take our input, encode it, and decode the result as a decimal object.

Answer 2

Answering my own question. I output Decimal objects surrounded by special character `. Then remove it and the double-quote from text.

class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            return '`'+str(o)+'`' #` is special, will be removed later
        return super(DecimalEncoder, self).default(o)

json.dumps(y, cls=DecimalEncoder).replace("\"`",'').replace("`\"",'')

Answer 3

From version 3.1 json library supports parse_float option that allows enconding/deconding as Decimal.

Refer to official documentation for usage examples.