import urandom

def uuid4():
    b = bytearray(urandom.getrandbits(8) for _ in range(16))

    # Set version (4) -> bits 12-15 of time_hi_and_version
    b[6] = (b[6] & 0x0F) | 0x40

    # Set variant (RFC 4122)
    b[8] = (b[8] & 0x3F) | 0x80

    def to_hex(x):
        return '{:02x}'.format(x)

    return (
        to_hex(b[0]) + to_hex(b[1]) + to_hex(b[2]) + to_hex(b[3]) + '-' +
        to_hex(b[4]) + to_hex(b[5]) + '-' +
        to_hex(b[6]) + to_hex(b[7]) + '-' +
        to_hex(b[8]) + to_hex(b[9]) + '-' +
        to_hex(b[10]) + to_hex(b[11]) + to_hex(b[12]) +
        to_hex(b[13]) + to_hex(b[14]) + to_hex(b[15])
    )