Using PyUSB with python 3.x .

C:\Python34\Scripts>pip3.4.exe install PyUSB
Downloading/unpacking PyUSB
  Running setup.py (path:C:\Users\mythcat\AppData\Local\Temp\pip_build_mythcat\PyUSB\setup.py) 
egg_info for package PyUSB

Installing collected packages: PyUSB
  Running setup.py install for PyUSB

Successfully installed PyUSB
Cleaning up...

Now you need to install this filter from here.
If not, you can get errors like this: raise NoBackendError('No backend available')
usb.core.NoBackendError: No backend available
Let's make a simple test example:

import usb.core
import usb.util
import sys

class find_class(object):
    def __init__(self, class_):
        self._class = class_
    def __call__(self, device):
        # first, let's check the device
        if device.bDeviceClass == self._class:
            return True
        # ok, transverse all devices to find an
        # interface that matches our class
        for cfg in device:
            # find_descriptor: what's it?
            intf = usb.util.find_descriptor(
                                        cfg,
                                        bInterfaceClass=self._class
                                )
            if intf is not None:
                return True

        return False

all = usb.core.find(find_all=1, custom_match=find_class(7))
print (all)

And show the result:

C:\Python34>python.exe usb_devs.py
< generator 0x0000000003d8d5e8="" at="" device_iter="" object="" >