Parses a string and returns a (port, gpio_bit) tuple.

def parse_port_pin(name_str):
    """Parses a string and returns a (port, gpio_bit) tuple."""
    if len(name_str) < 3:
        raise ValueError("Expecting pin name to be at least 3 characters")
    if name_str[:2] != 'GP':
        raise ValueError("Expecting pin name to start with GP")
    if not name_str[2:].isdigit():
        raise ValueError("Expecting numeric GPIO number")
    port = int(int(name_str[2:]) / 8)
    gpio_bit = 1 << int(int(name_str[2:]) % 8)
    return (port, gpio_bit)