How to write Json null value as an empty line in new file (converting json based log into column format, i.e., one file per column)

Note, the input file is actually an NDJSON. See the docs.

That being said, since furas already gave an excellent answer on how to process the NDJSON logfile I'm going to skip that part. Do note that there's a library to deal with NDJSON files. See PyPI.

His code needs minimal adjustment to deal with the undefined edge case. The null value is a valid JSON value, so his code doesn't break on that.

You can fix this easily by a string.replace() while doing the json.loads() so it becomes valid JSON, and then you can check while writing if value == None to replace the value with an empty string. Note that None is the python equivalent of JSON's null.

Please note the inclusion of : in the replace function, it's to prevent false negatives...

main loop logic

for line in file_obj:
    # the replace function makes it valid JSON
    data = json.loads(line.replace(': undefined', ': null'))
    print(data)
    process_dict(data, write_func)

write_func() function adjustment

def write_func(key, value):
    with open(key + '.column', "a") as f:
        # if the value == None, make it an empty string.
        if value == None:
            value = ''
        f.write(str(value) + "\n")

I used the following as the input string:

{"timestamp": "2022-01-14T00:12:21.000", "Field1": 10, "Field_Doc": {"f1": 0}}
{"timestamp": "2022-01-18T00:15:51.000", "Field_Doc": {"f1": 0, "f2": 1.7, "f3": 2}}
{"timestamp": "2022-01-14T00:12:21.000", "Field1": null, "Field_Doc": {"f1": undefined}}