format-ndjson.py

import json
import os.path
import sys

if __name__ == "__main__":
    if len(sys.argv) != 2:
        sys.exit(f"Usage: {os.path.basename(sys.argv[0])} FILE")

    file_name = sys.argv[1]

    try:
        with open(file_name) as f:
            for lineno, line in enumerate(f, start=1):
                try:
                    j = json.loads(line)
                except ValueError:
                    print(
                        f"Warning: could not parse line {lineno} as JSON",
                        file=sys.stderr,
                    )
                    continue

                print(json.dumps(j, indent=4))
    except FileNotFoundError:
        sys.exit(f"File '{file_name}' not found")
    except:
        sys.exit(f"An unexpected error occurred: {sys.exc_info()[1]}")