How to pass parameters to a custom JSONEncoder default() function in Python

Question

In my Flask application, I use a custom JSONEncoder that serializes decimal.Decimal objects rounded to two places.

class MyJsonEncoder(JSONEncoder):

    def default(self, obj, prec=2):

        if isinstance(obj, Decimal):
            return str(obj.quantize(Decimal('.'+'0'*(prec-1)+'2')))
        else:
            return JSONEncoder.default(self, obj)

The prec parameter lets me change the precision of the rounding. It defaults to two places. I want to occasionally call json.dumps and pass it a prec parameter so I can force the decimal.Decimal objects to round to 4 places instead.

json_string = json.dumps(some_data, prec=4)

But when I do this, the JSON module throws:

TypeError: __init__() got an unexpected keyword argument 'prec'

Is it possible to do what I'm trying here? I don't understand why the JSON module is doing anything with the **kwargs. Can I force it to ignore them?

Alex Hall · Accepted Answer · 2018-04-27 22:17:14Z

3

class MyJsonEncoder(JSONEncoder):

    def __init__(self, prec=2, **kwargs):
        super(MyJsonEncoder, self).__init__(**kwargs)
        self.prec = prec

    def default(self, obj):
        prec = self.prec
        ...

MyJsonEncoder(prec=4).encode(some_data)

answered Apr 27, 2018 at 22:17

Alex Hall

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5 Comments

susodapop Over a year ago

Thanks for editing your response. Your initial response sent me in the right direction and I ended up writing something exactly like what you have above. Works like a charm!

Campa Feb 7 at 16:22

How would you use that in a json.dump call to serialize to file with indentation etc.?

Alex Hall Feb 7 at 16:33

@Campa using json.dump(..., cls=MyJsonEncoder).

Campa Feb 7 at 16:55

right, but this way you cannot parameterize the value of prec, right?

Alex Hall Feb 7 at 16:58

@Campa json.dump(..., cls=MyJsonEncoder, prec=4)

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