Write two-dimensional list to JSON file

I thought you could use my answer to another similar question to do what you want. While it works with json.dumps(), you pointed out that it didn’t for some reason with json.dump().

After looking into the matter, I discovered that the encode() method of the derived json.JSONEncoder class that was being overridden in the linked answer, is only called when dumps() is called, but not when dump() is called.

Fortunately, I was able to determine that the iterencode() method does get invoked in both cases — so was able to fix the problem by more-or-less simply moving the code from encode() to iterencode().

The code immediately below is a revised version with this change in it:

Modified version of the code in my answer to other question:

from _ctypes import PyObj_FromPtr  # see https://stackoverflow.com/a/15012814/355230
import json
import re


class NoIndent(object):
    """ Value wrapper. """
    def __init__(self, value):
        if not isinstance(value, (list, tuple)):
            raise TypeError('Only lists and tuples can be wrapped')
        self.value = value


class MyEncoder(json.JSONEncoder):
    FORMAT_SPEC = '@@{}@@'  # Unique string pattern of NoIndent object ids.
    regex = re.compile(FORMAT_SPEC.format(r'(\d+)'))  # compile(r'@@(\d+)@@')

    def __init__(self, **kwargs):
        # Keyword arguments to ignore when encoding NoIndent wrapped values.
        ignore = {'cls', 'indent'}

        # Save copy of any keyword argument values needed for use here.
        self._kwargs = {k: v for k, v in kwargs.items() if k not in ignore}
        super(MyEncoder, self).__init__(**kwargs)

    def default(self, obj):
        return (self.FORMAT_SPEC.format(id(obj)) if isinstance(obj, NoIndent)
                    else super(MyEncoder, self).default(obj))

    def iterencode(self, obj, **kwargs):
        format_spec = self.FORMAT_SPEC  # Local var to expedite access.

        # Replace any marked-up NoIndent wrapped values in the JSON repr
        # with the json.dumps() of the corresponding wrapped Python object.
        for encoded in super(MyEncoder, self).iterencode(obj, **kwargs):
            match = self.regex.search(encoded)
            if match:
                id = int(match.group(1))
                no_indent = PyObj_FromPtr(id)
                json_repr = json.dumps(no_indent.value, **self._kwargs)
                # Replace the matched id string with json formatted representation
                # of the corresponding Python object.
                encoded = encoded.replace(
                            '"{}"'.format(format_spec.format(id)), json_repr)

            yield encoded

Applying it to your question:

# Example of using it to do get the results you want.

alfa = [('a','b','c'), ('d','e','f'), ('g','h','i')]
data = [(1,2,3), (2,3,4), (4,5,6)]

data_struct = {
    'data': [NoIndent(elem) for elem in data],
    'alfa': [NoIndent(elem) for elem in alfa],
}

print(json.dumps(data_struct, cls=MyEncoder, sort_keys=True, indent=4))

# Test custom JSONEncoder with json.dump()
with open('data_struct.json', 'w') as fp:
    json.dump(data_struct, fp, cls=MyEncoder, sort_keys=True, indent=4)
    fp.write('\n')  # Add a newline to very end (optional).

Resulting output:

{
    "alfa": [
        ["a", "b", "c"],
        ["d", "e", "f"],
        ["g", "h", "i"]
    ],
    "data": [
        [1, 2, 3],
        [2, 3, 4],
        [4, 5, 6]
    ]
}