Scanning for available USB tty devices on OSX differes slightly from Linux.

scanosx.py

#adapted from pySerial examples for use on OSX systems
import serial
from glob import glob

def _scan():
    dev_list  = glob('/dev/tty*usb*')
    available = []
    for dev in dev_list:
        try:
            s = serial.Serial(dev)
            available.append(dev)
            s.close()
        except serial.SerialException:
            pass
    return available