how to convert null to empty string when encode json in python

Question

In the backend django I will return data to frond end as json, but in frondend there are many "null" value, and one solution is to change code to convert value None to empty string "" when create the value, but since there are many places, I'd like to write a customized encoding function to convert None to "" when dump the obj into string. I used the following class, but it doesn't work:

class PythonObjectEncoder(json.JSONEncoder):
    def encode(self, obj):
        if obj is None:
            obj = ''
        return json.JSONEncoder().encode(obj)

data = [1,2,3,None,
        [[[2],3],4]
        ]

j = json.dumps(data, cls=PythonObjectEncoder)

Chen Gupta · Accepted Answer · 2017-04-28 11:51:41Z

1

One way to do this is just to filter the json object beforehand. For example, with such function:

def fix_data(data):
    if type(data) is list:
        for i, e in enumerate(data):
            if e is None:
                data[i] = ''
            else:
                fix_data(e)

    elif type(data) is dict:
        for k, v in data.items():
            if v is None:
                data[k] = ''
            else:
                fix_data(v)

# Example:
data = ['1', None, '0', {'test' : None}]
fix_data(data)
print(data)

answered Apr 28, 2017 at 11:51

Chen Gupta

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Comments

Morreski · Accepted Answer · 2017-04-28 13:22:56Z

It is not possible to tell the json decoder something like:

Please encode this piece of data but replace each occurence of None by an empty string instead of null.

The reason is that this mapping is defined in json.scanner and can not be altered.

However, if you know what your data look like before json-encoding. You can pre-process them before encoding. Here is a piece of code that matches your example:

import json

class CustomListEncoder(json.JSONEncoder):
    def encode(self, o):
        if not isinstance(o, list):
            raise TypeError("This encoder only handle lists")
        none_to_empty_str = map(lambda x: "" if x is None else x, o)
        return super().encode(list(none_to_empty_str))

json.dumps([1,2,None, [3, 4, 5]], cls=CustomListEncoder)

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