pack_unpack_file.py

def pack_file(file_name, outfile_name, no_of_bytes_to_prefix=100):
    file_name_encoded = bytes(file_name, encoding='utf-8')
    file_name_length = len(file_name_encoded)
    if file_name_length > no_of_bytes_to_prefix:
        raise Exception('file name after utf-8 encoding is too long')
    prefixed_bytes = bytearray(no_of_bytes_to_prefix)
    prefixed_bytes[0:file_name_length] = file_name_encoded
    
    chunk_buffer = 4096
    with open(file_name, 'rb') as infile, open(outfile_name, 'wb') as outfile:
        outfile.write(prefixed_bytes)
        while True:
            chunk = infile.read(chunk_buffer)
            if not chunk:
                break
            outfile.write(chunk)


def unpack_file(file_name, no_of_bytes_prefixed=100):
    import os.path
    
    chunk_buffer = 4096
    with open(file_name, 'rb') as infile:
        prefixed_bytes = infile.read(no_of_bytes_prefixed)
        idx = prefixed_bytes.find(0)
        if idx == -1:
          idx = len(prefixed_bytes)
        actual_file_name = prefixed_bytes[:idx].decode('utf-8')
        if os.path.isfile(actual_file_name):
            return 1, Exception(f'Output file [{actual_file_name}] already exists')
        if os.path.isdir(actual_file_name):
            return 2, Exception(f'A directory with name [{actual_file_name}] exists')
        else:
            with open(actual_file_name, 'wb') as outfile:
              while True:
                  chunk = infile.read(chunk_buffer)
                  if not chunk:
                      break
                  outfile.write(chunk)
            return 0, actual_file_name