PyUSB ValueError:没有可用的后端

问题描述

我在 Win 7 操作系统上运行 Python 2.7.8.我正在尝试通过 PyUSB 与 USB 设备(Numato 32 通道 GPIO 设备)通信.

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.

我从以下网址下载了 walac-pyusb-7071ad3:http://walac.github.io/pyusb

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

我停止收到ValueError:没有可用的后端".有哪位 Python 专家能告诉我哪里错了吗?

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

代码如下:

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)

这是 Python 错误消息:

Here is Python error message:

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

推荐答案

我也有同样的错误，但是我没有成功使用 find_library(TypeError: get_backend() got an意外的关键字参数find_library").我想，虽然你没有说，backend 是无效的(None).

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).

C:Python27 路径中是否有 libusb1 实现?我想你没有将它安装在 Python 的文件夹中，如果是这样，那就是你的答案:PyUSB 后端不可访问.

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.

否则，在不使用 find_library 的情况下，您必须在 PATH 环境变量中提供 libusb1 实现.我是这样做的(您可以将 os.getcwd() 替换为您的位置):

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

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