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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _read_signer(key_filename):
 """Reads the given file as a hex key. Args: key_filename: The filename where the key is stored. If None, defaults to the default key for the current user. Returns: Signer: the signer Raises: CliException: If unable to read the file. """

    filename = key_filename
    if filename is None:
        filename = os.path.join(os.path.expanduser('~'),
                                '.sawtooth',
                                'keys',
                                getpass.getuser() + '.priv')

    try:
        with open(filename, 'r') as key_file:
            signing_key = key_file.read().strip()
    except IOError as e:
        raise CliException('Unable to read key file: {}'.format(str(e)))

    try:
        private_key = Secp256k1PrivateKey.from_hex(signing_key)
    except ParseError as e:
        raise CliException('Unable to read key in file: {}'.format(str(e)))

    context = create_context('secp256k1')
    crypto_factory = CryptoFactory(context)
    return crypto_factory.new_signer(private_key)