def decode_varint_py(buffer, pos=0):
    """ Decode an integer from a varint presentation. See
    https://developers.google.com/protocol-buffers/docs/encoding?csw=1#varints
    on how those can be produced.

        Arguments:
            buffer (bytearry): buffer to read from.
            pos (int): optional position to read from

        Returns:
            (int, int): Decoded int value and next read position
    """
    result = buffer[pos]
    if not (result & 0x81):
        return (result >> 1), pos + 1
    if not (result & 0x80):
        return (result >> 1) ^ (~0), pos + 1

    result &= 0x7f
    pos += 1
    shift = 7
    while 1:
        b = buffer[pos]
        result |= ((b & 0x7f) << shift)
        pos += 1
        if not (b & 0x80):
            return ((result >> 1) ^ -(result & 1), pos)
        shift += 7
        if shift >= 64:
            raise ValueError("Out of int64 range")