Convert input JSON to NDJSON (newline delimited JSON)

json_to_ndjson.py

#! /usr/bin/env python3

import json
import sys


def json_to_ndjson(input_file: str, output_file=None):
    """Converts a JSON input file of JSON objects to NDJSON format.

    Args:
        input_file: A JSON file of JSON objects.
        output_file: (Optional) The path to the output NDJSON file. If None,
                     the NDJSON string is returned.

    Returns:
        If output_file is None, returns the NDJSON string. Otherwise, returns None (and creates output file)
    """
    try:
        with open(input_file, 'r') as f:
            json_data = json.load(f)
    except FileNotFoundError:
        print(f"Error: File not found at {input_file}")
        sys.exit(1)
    except json.JSONDecodeError:
        print(f"Error: Invalid JSON format in {input_file}")
        sys.exit(2)
    except Exception as e:
        print(f"An unexpected error occurred: {e}")
        sys.exit(10)

    if isinstance(json_data, list):
        ndjson_lines = [json.dumps(item) for item in json_data]
    elif isinstance(json_data, dict):
        ndjson_lines = [json.dumps(json_data)]
    else:
        raise ValueError("Input must be a JSON object (dict) or a list of JSON objects.")

    ndjson_string = "\n".join(ndjson_lines)

    if output_file:
        with open(output_file, "w") as f:
            f.write(ndjson_string)
        return None
    else:
        print(ndjson_string)
        return ndjson_string


if __name__ == "__main__":
    if len(sys.argv) == 2:
        input_file = sys.argv[1]
        json_to_ndjson(input_file)
    elif len(sys.argv) == 3:
        input_file = sys.argv[1]
        output_file = sys.argv[2]
        json_to_ndjson(input_file, output_file)
    else:
        print("Usage: json_to_ndjson INPUT_FILE [OUTPUT_FILE]")