PyUSB ValueError: No backend available

问题:

I am runing Python 2.7.8 at Win 7 operation system. I am trying to communicate a USB device (Numato 32 channel GPIO device) by PyUSB.

I downloaded walac-pyusb-7071ad3 from URL: http://walac.github.io/pyusb

I stop at receiving "ValueError: No backend available". Could any Python expert tell me where is wrong?

Here is the code:

import sys
import usb
import usb.core
import usb.util
import usb.backend.libusb1

backend = usb.backend.libusb1.get_backend(find_library=lambda C:'\Python27')
numato = usb.core.find(idVendor=00000006,idProduct = 00000000, backend=backend)

Here is Python error message:

Traceback (most recent call last):
  File "C:\Python_Yang\PyUSBNumato.py", line 19, in <module>
    numato = usb.core.find(idVendor=00000006,idProduct = 00000000, backend=backend)
  File "C:\Python_Yang\usb\core.py", line 1199, in find
    raise ValueError('No backend available')
ValueError: No backend available

回答1:

I had the same error, but I didn't succeed to use find_library(TypeError: get_backend() got an unexpected keyword argument 'find_library'). I suppose, although you did not say it, that backend is not valid (None).

Do you have the libusb1 implementation in the path C:\Python27 ? I suppose you didn't install it in the Python's folder and if so, there's your answer: PyUSB backend not accessible.

Otherwise, without using find_library, you must have the libusb1 implementation available in the PATH environment variable. I did it like this (you can replace os.getcwd() with your location):

def get_backend_libusb01():
    libusb01_location = os.getcwd()

    # load-library (ctypes.util.find_library) workaround: also search the current folder
    is_current_folder_in_search_path = True
    if None == usb.backend.libusb0.get_backend():
        is_current_folder_in_search_path = libusb01_location in os.environ['PATH']
        if not is_current_folder_in_search_path:
            os.environ['PATH'] += os.pathsep + libusb01_location

    backend = usb.backend.libusb0.get_backend()

    if not is_current_folder_in_search_path:
        os.environ['PATH'] = os.environ['PATH'].replace(os.pathsep + libusb01_location, "")

    return backend

回答2:

I had this trouble, and i switched python libusb wrappers and its gone: https://github.com/vpelletier/python-libusb1

回答3:

I did: - Download and install libusb-win32-devel-filter-1.2.6.0.exe. It should work.

From:

Pyusb on windows - no backend available

And really works for me.