如何在Python中将参数传递给自定义JSONEncoder default（）函数

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?

Answer 1

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)