<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def hash(filepath, method='sha1', buffer_size=65536):
 """ Calculate a hash of a local file. Parameters filepath : str method : {'sha1', 'md5'} buffer_size : int, optional (default: 65536 byte = 64 KiB) in byte Returns ------- hash : str """

    if method == 'sha1':
        hash_function = hashlib.sha1()
    elif method == 'md5':
        hash_function = hashlib.md5()
    else:
        raise NotImplementedError('Only md5 and sha1 hashes are known, but '
                                  ' \'{}\' was specified.'.format(method))

    with open(filepath, 'rb') as fp:
        while True:
            data = fp.read(buffer_size)
            if not data:
                break
            hash_function.update(data)
    return hash_function.hexdigest()