python - ValueError: No backend available -

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i try use acr122u , nfcpy library/module python 2.7 on mac os x 10.9.5.

i followed instruction on site: http://nfcpy.readthedocs.org/en/latest/topics/get-started.html still error when typing:

clf = nfc.contactlessfrontend('usb')  traceback (most recent call last):   file "<pyshell#1>", line 1, in <module>     clf = nfc.contactlessfrontend('usb')   file "nfc/clf.py", line 105, in __init__     if path , not self.open(path):   file "nfc/clf.py", line 156, in open     self.dev = nfc.dev.connect(path)   file "nfc/dev/__init__.py", line 55, in connect     found = transport.usb.find(path)   file "nfc/dev/transport.py", line 169, in find     d in cls.usb_core.find(find_all=true, **match)]   file "/library/frameworks/python.framework/versions/2.7/lib/python2.7/site-packages/usb/core.py", line 1199, in find     raise valueerror('no backend available') valueerror: no backend available

any ideas?


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