@classmethod
def from_private_key_file(cls, filename, password=None):
    """
        Create a key object by reading a private key file.  If the private
        key is encrypted and ``password`` is not ``None``, the given password
        will be used to decrypt the key (otherwise `.PasswordRequiredException`
        is thrown).  Through the magic of Python, this factory method will
        exist in all subclasses of PKey (such as `.RSAKey` or `.DSSKey`), but
        is useless on the abstract PKey class.

        .. note::
            It is recommended to use the key-type-agnostic
            `.load_private_key_file` function instead.

        :param str filename: name of the file to read
        :param str password:
            an optional password to use to decrypt the key file, if it's
            encrypted
        :return: a new `.PKey` based on the given private key

        :raises: ``IOError`` -- if there was an error reading the file
        :raises: `.PasswordRequiredException` -- if the private key file is
            encrypted, and ``password`` is ``None``
        :raises: `.SSHException` -- if the key file is invalid
        """
    key = cls(filename=filename, password=password)
    return key
