Re: [pyusb-users] Compatibility with Python 3.3?

[Xiaofan C. <xia...@gm...>]

On Tue, Jan 22, 2013 at 1:50 PM, Xiaofan Chen <xia...@gm...> wrote:
> On Thu, Jan 10, 2013 at 3:43 PM, Cybergibbons <cyb...@gm...> wrote:
>> I have been trying to use PyUSB for a few hours using Python 3.3 under
>> Windows 7 and was getting issues with "Cannot find backend".
>> ctypes.util.find_library was failing to find the DLL, yet if I stepped
>> through the code, it would find the DLL. Even once this was resolved, a
>> problem occurred in the init code.
>
> Seem to be okay for me. I just installed Python 3.3 and pywin32
> and I can find the device with Python 3.3.
>
> Which backend are you using? I am trying out the libusb-1.0
> backend and it seems to work. The device is ST-Link V2 and
> it uses WinUSB driver.
>
> Python 3.3.0 (v3.3.0:bd8afb90ebf2, Sep 29 2012, 10:55:48) [MSC v.1600 32 bit (In
> tel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>>>> import usb.core
>>>> import usb.util
>>>> dev = usb.core.find(idVendor=0x0483, idProduct=0x3748)
>>>> dev
> <usb.core.Device object at 0x00CF9E90>
>>>>

Hmm, I was putting libusb-1.0.dll under C:\Python33 for the above
testing. Once I move it to C:\Windows\System32, I will have
the same problem as you. Python 2.7 is okay.

Python 3.3.0 (v3.3.0:bd8afb90ebf2, Sep 29 2012, 10:55:48) [MSC v.1600 32 bit (In
tel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import usb.core
>>> import usb.util
>>> dev = usb.core.find(idVendor=0x0483, idProduct=0x3748)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python33\lib\site-packages\usb\core.py", line 861, in find
    raise ValueError('No backend available')
ValueError: No backend available

Python 2.7.3 (default, Apr 10 2012, 23:31:26) [MSC v.1500 32 bit (Intel)] on win
32
Type "help", "copyright", "credits" or "license" for more information.
>>> import usb.core
>>> import usb.util
>>> dev = usb.core.find(idVendor=0x0483, idProduct=0x3748)
>>> dev
<usb.core.Device object at 0x00C345B0>
>>>


So it seems to me maybe the DLL finding method in Python 3.3
is a bit different.

Google seems to suggest this is a Python 3.3 bug.
http://bugs.python.org/issue16283
Title:	ctypes.util.find_library does not find all DLLs anymore
Type:	behavior	Stage:	
Components:	ctypes	Versions:	Python 3.3


-- 
Xiaofan