Python PyUSB ValueError：没有可用的后端
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PyUSB ValueError: No backend available
提问by Joyful CD
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.
我在 Win 7 操作系统上运行 Python 2.7.8。我正在尝试通过 PyUSB 通信 USB 设备（Numato 32 通道 GPIO 设备）。
I downloaded walac-pyusb-7071ad3 from URL: http://walac.github.io/pyusb
我从 URL 下载了 walac-pyusb-7071ad3：http://walac.github.io/pyusb
I stop at receiving "ValueError: No backend available". Could any Python expert tell me where is wrong?
我停止接收“ValueError：没有可用的后端”。有没有 Python 专家能告诉我哪里错了？
Here is the code:
这是代码：
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:
这是 Python 错误消息：
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by Liviu
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 backendis not valid (None).
我有同样的错误，但我没有成功使用find_library( TypeError: get_backend() got an unexpected keyword argument 'find_library')。我想，虽然你没有说，但那backend是无效的（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.
你在路径中有 libusb1 实现C:\Python27吗？我想你没有将它安装在 Python 的文件夹中，如果是这样，你的答案是：PyUSB backend not access。
Otherwise, without using find_library, you must have the libusb1 implementation available in the PATHenvironment variable. I did it like this (you can replace os.getcwd()with your location):
否则，在不使用的情况下find_library，您必须在PATH环境变量中提供 libusb1 实现。我是这样做的（你可以os.getcwd()用你的位置替换）：
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by Hayden Thring
I had this trouble, and i switched python libusb wrappers and its gone: https://github.com/vpelletier/python-libusb1
我遇到了这个问题，我切换了 python libusb 包装器，它不见了：https: //github.com/vpelletier/python-libusb1
回答by Wellington1993
I did: - Download and install libusb-win32-devel-filter-1.2.6.0.exe. It should work.
我做了： - 下载并安装 libusb-win32-devel-filter-1.2.6.0.exe。它应该工作。
From:
从：
Pyusb on windows - no backend available
And really works for me.
而且真的对我有用。
