Matroska (mkv) support

[Tophness]

Successfully tested read and write functionality, all up to spec with the RFC standard.
#3 #575

[Tophness]

Read test:

import sys
from mutagen.mkv import MKVFile


def test_mkv_file(file_path):
    if not os.path.exists(file_path):
        print(f"Error: File not found: {file_path}")
        return

    file_size = os.path.getsize(file_path)
    print(f"\nTesting file: {file_path} ({file_size} bytes)")
    print("=" * 80)

    try:
        mkv = MKVFile(file_path)
        print("\nReading metadata:")
        print_metadata(mkv)
    except Exception as e:
        print(f"Error reading {file_path}: {e}")
        import traceback
        traceback.print_exc()


def _print_value(value, indent_level):
    indent = " " * indent_level
    if isinstance(value, bytes):
        try:
            value_str = value.decode('utf-8', errors='replace')
            print(f"{indent}{value_str}")
        except Exception:
            print(f"{indent}0x{value.hex()}")
    else:
        print(f"{indent}{value}")


def print_metadata(mkv):
    if not mkv.tags:
        print("No metadata found in the file.")
        return

    print("Metadata found:")
    for key, value in sorted(mkv.tags.items()):
        if isinstance(value, list):
            print(f"  {key}:")
            for item in value:
                _print_value(item, indent_level=4)
        else:
            print(f"  {key}: ", end="")
            _print_value(value, indent_level=0)


if __name__ == "__main__":
    if len(sys.argv) > 1:
        for file_path in sys.argv[1:]:
            test_mkv_file(file_path)
    else:
        print("Please provide MKV file paths as arguments.")
        print("Usage: python test_mkv.py [file1.mkv file2.mkv ...]")```

[Tophness]

Write test:

import shutil
from mutagen.mkv import MKVFile

def test_mkv_write():
    input_file = r"input.mkv"
    output_file = r"output.mkv"

    if not os.path.exists(input_file):
        print(f"Error: Input file not found at '{input_file}'")
        print("Please ensure the file exists or modify the 'input_file' variable.")
        return

    # --- Test Execution ---
    print(f"Using input file: {input_file}")
    print(f"Will create and modify: {output_file}\n")

    # 1. Copy the file to avoid modifying the original
    try:
        shutil.copyfile(input_file, output_file)
        print(f"Successfully copied '{input_file}' to '{output_file}'.")
    except Exception as e:
        print(f"Error copying file: {e}")
        return

    # 2. Read the original comment tag
    try:
        mkv = MKVFile(output_file)
        original_comment = mkv.tags.get('COMMENT')
        if original_comment:
            print(f"Original 'COMMENT' tag: '{original_comment[0]}'")
        else:
            print("No 'COMMENT' tag found in the original file.")
    except Exception as e:
        print(f"Error reading tags from '{output_file}': {e}")
        return

    # 3. Set a new comment tag and save the file
    new_comment_value = "test"
    print(f"\nSetting 'COMMENT' tag to: '{new_comment_value}'")
    try:
        mkv.tags['COMMENT'] = new_comment_value
        mkv.save()
        print("Successfully saved the file with the new tag.")
    except Exception as e:
        print(f"Error saving file '{output_file}': {e}")
        return

    # 4. Read the file again to verify the change
    print("\nVerifying the change...")
    try:
        mkv_verify = MKVFile(output_file)
        written_comment = mkv_verify.tags.get('COMMENT')
        
        if written_comment:
            print(f"Read back 'COMMENT' tag: '{written_comment[0]}'")
            if written_comment[0] == new_comment_value:
                print("\n--- VERIFICATION SUCCESSFUL ---")
                print("The 'COMMENT' tag was written and read back correctly.")
            else:
                print("\n--- VERIFICATION FAILED ---")
                print(f"Expected '{new_comment_value}' but got '{written_comment[0]}'")
        else:
            print("\n--- VERIFICATION FAILED ---")
            print("The 'COMMENT' tag was not found after saving.")
            
    except Exception as e:
        print(f"Error during verification read of '{output_file}': {e}")

if __name__ == "__main__":
    test_mkv_write()