Richard Musil wrote:
> I have found myself in an awkward situation with current (Python 3.7) JSON
> module. Basically it boils down to how it handles floats. I had been hit
> on this particular case:
>
> In [31]: float(0.6441726684570313)
> Out[31]: 0.6441726684570312
>
> but I guess it really does not matter.
>
> What matters is that I did not find a way how to fix it with the standard
> `json` module. I have the JSON file generated by another program (C++
> code, which uses nlohmann/json library), which serializes one of the
> floats to the value above. Then when reading this JSON file in my Python
> code, I can get either decimal.Decimal object (when specifying
> `parse_float=decimal.Decimal`) or float. If I use the latter the least
> significant digit is lost in deserialization.
>
> If I use Decimal, the value is preserved, but there seems to be no way to
> "serialize it back". Writing a custom serializer:
>
> class DecimalEncoder(json.JSONEncoder):
> def default(self, o):
> if isinstance(o, decimal.Decimal):
> return str(o) # <- This becomes quoted in the serialized
> output
> return super.default(o)
>
> seems to only allow returning "string" value, but then serializes it as a
> string! I.e. with the double quotes. What seems to be missing is an
> ability to return a "raw textual representation" of the serialized object
> which will not get mangled further by the `json` module.
You have overwritten the wrong method:
$ cat demo.py
import json
import decimal
class DecimalEncoder(json.JSONEncoder):
def encode(self, o):
if isinstance(o, decimal.Decimal):
return str(o)
return super().encode(o)
input = "0.6441726684570313"
obj = json.loads(input, parse_float=decimal.Decimal)
output = json.dumps(obj, cls=DecimalEncoder)
assert input == output
$ python3 demo.py
$
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