get_file_part.py

def get_file_part(file_path, specifier):
    size = file_path.stat().st_size

    if specifier.isdigit():
        specifier = int(specifier)
        if specifier > 0:
            with file_path.open('rb') as file:
                return file.read(specifier)
        else:
            with file_path.open('rb') as file:
                file.seek(max(size + specifier, 0))
                return file.read(-specifier)
    elif specifier.endswith('%'):
        percent = float(specifier[:-1])
        bytes_to_read = int(size * (percent / 100))
        if specifier.startswith('-'):
            with file_path.open('rb') as file:
                file.seek(max(size - bytes_to_read, 0))
                return file.read(bytes_to_read)
        else:
            with file_path.open('rb') as file:
                return file.read(bytes_to_read)
    elif '-' in specifier:
        start, end = map(int, specifier.split('-'))
        if end == 0:
            end = size
        with file_path.open('rb') as file:
            file.seek(start)
            return file.read(end - start)
    else:
        return None