Dump given JSON document to a string, flattening lists (recursively) but respecting the indent for other objects.

json_dumps_compact.py

import json


def json_dumps_compact(data, indent: int | str, **kwargs) -> str:
    """
    Dump given JSON document to a string, flattening lists (recursively) but respecting the indent for other objects.

    :param data: JSON object to be dumped
    :param indent: indent level for JSON object members (None is not supported)
    :param kwargs: other arguments passed to :func:`json.dumps` ("separators" is not supported)
    :return: resulting string
    """
    if indent is None:
        raise ValueError("Unsupported argument 'indent=None' given")
    if kwargs.get("separators") is not None:
        raise ValueError("Unsupported argument 'separators' given")

    # The "not" cases are necessary because empty lists/dicts by default end up with both brackets on the same line
    is_list_start = lambda line_: line_.endswith("[")
    is_list_end = lambda line_: line_.endswith(("],", "]")) and not line_.endswith(("[],", "[]"))
    is_dict_start = lambda line_: line_.endswith("{")
    is_dict_end = lambda line_: line_.endswith(("},", "}")) and not line_.endswith(("{},", "{}"))
    at_bound = lambda lo, up: is_list_start(lo) or is_dict_start(lo) or is_list_end(up) or is_dict_end(up)

    lines = json.dumps(data, indent=indent, **kwargs).split("\n")
    processed_lines = []
    current_line = []
    lists_to_close = 0
    for line in lines:
        # If a list starts, lift all following lines onto the same line until it is closed;
        # strip indents for all except first start of a list
        if is_list_start(line):
            current_line.append(line if not lists_to_close else line.strip())
            lists_to_close += 1
        elif is_list_end(line):
            lists_to_close -= 1
            current_line.append(line.strip())
            if not lists_to_close:
                # Final list is closed: assemble line; provide separators only between actual items
                seps = ("" if at_bound(lo, up) else " " for lo, up in zip(current_line[:-1], current_line[1:]))
                processed_lines.append("".join(i for t in zip(current_line, seps) for i in t) + current_line[-1])
                current_line = []
        else:
            if lists_to_close:
                current_line.append(line.strip())  # Inside list → append to current line
            else:
                processed_lines.append(line)  # Append to processed lines
    return "\n".join(processed_lines)