Re: [pyusb-users] No Backend Available

[Jonas M. <jo...@pr...>]

On Wed, Jan 20, 2021 at 8:04 PM Steve Kranish <skr...@ve...> wrote:
>
>
> On 1/20/2021 4:41 PM, Jonas Malaco wrote:
> > On Wed, Jan 20, 2021 at 6:20 PM Steve Kranish via pyusb-users
> > <pyu...@li...> wrote:
> >> This is a widely reported problem which has just been left dangling for
> >> many people. I have yet to see a complete, meaningful resolution.
> > This is not true.
> >
> > The solution, as you have already gathered, is to install libusb.
>
> As mentioned below, I have installed libusb 1.0.23b7
>
> It installs 32 and 64 bit versions, or at least 32bit and 64 bit
> directories. The DLLs ARE different.
>
> I have copied the 64 bit library to c:\Windows\System32 AND my python
> working directory, where the py files are.
>
>
> > By the way, you haven't mentioned whether you're running 32 or 64 bit
> > Windows and Python.
> Windows 7 - 64 bit, Python 3.85, 64 bit
> > I have libusb-1.0.dll in c:\windows\system32 AND the directory where my
> >> python script exits. I still get the “No backend available” error.
> > Once common 64-bit Windows problem is that 64-bits DLLs go in
> > C:\Windows\System32 while 32-bit DLLs need to go in
> > C:\Windows\SysWOW64.
> >
> > Perhaps that's your issue?
>
> The 'library installation' directory is
>
> C:\Users\skranish\AppData\Local\Programs\Python\Python38\lib\site-packages\libusb
>
> I have used MS Process Monitor to see if libusb-1.0.dll is ever
> accessed. Python does look for it (QueryDirectory) in
>
> C:\Users\skranish\AppData\Local\Programs\Python\Python38\lib\site-packages\libusb
>
> and my working directory, but there is nothing in the log that indicates
> the file was ever accessed. It never looked in System32 or SysWOW64
>
> Nonetheless, I put the 32 bit library in C:\Windows\SysWOW64.
>
> and the 64 bit library in C:\Windows\System32.
>
> I think I already had it that way.
>
> It does seem bass-ackwards to me.
>
>
> Perhaps this is of note? pyusb 1.1.0 should be the 64bit version

PyUSB itself is architecture-agnostic; but the Python interpreter can
be 32 or 64 bit.


> pyUsb installs in
>
> C:\Users\skranish\AppData\Local\Programs\Python\Python38\lib\site-packages\pyusb-1.1.0.dist-info
>
> The files in this directory are:
>
>
>   Directory of
> C:\Users\skranish\AppData\Local\Programs\Python\Python38\lib\site-packages\pyusb-1.1.0.dist-info
>
> 01/17/2021  04:53 PM    <DIR>          .
> 01/17/2021  04:53 PM    <DIR>          ..
> 01/17/2021  04:53 PM                 4 INSTALLER
> 01/17/2021  04:53 PM             1,513 LICENSE
> 01/17/2021  04:53 PM             2,176 METADATA
> 01/17/2021  04:53 PM             2,340 RECORD
> 01/17/2021  04:53 PM                 0 REQUESTED
> 01/17/2021  04:53 PM                 4 top_level.txt
> 01/17/2021  04:53 PM                92 WHEEL
>                 7 File(s)          6,129 bytes
>
> That is all!
>
> After copying the dll again (as above), and running again, I find that
> it DOES find a backend.

Happy to hear!


> According to Process Explorer, there is now an QueryNameInformationFile for:
>
> C:\Windows\System32\libusb-1.0.dll  (because the program explicitly
> looks for it there!)
>
> in addition to accessing libusb-1.0.dll in the 'library install' directory.
>
> Perhaps I had the wrong version of the library somewhere?

Yes, that was probably it.

I'm not super familiar with how DLLs are loaded by Python on Windows, but it's
possible that if a DLL is found with the correct *name* the search stops, even
if later the DLL turns out to have been built for the wrong architecture.


> Thanks
>
> Steve
>
> import usb.core
> import usb.util
>
> from infi.devicemanager import DeviceManager
> dm = DeviceManager()
> devices = dm.all_devices
> for i in devices:
>      try:
>          print ('{} : address: {}, bus: {}, location:
> {}'.format(i.friendly_name, i.address, i.bus_number, i.location))
>      except Exception:
>          pass
>
>
> import usb.backend.libusb1
>
> backend = usb.backend.libusb1.get_backend(find_library=lambda x:
> "C:\\Windows\\System32\\libusb-1.0.dll")
> print(backend)
> dev = usb.core.find(backend=backend, find_all=True)
> print(dev)
>
> def EnumerateUSB():    #I use a simple function that scans all known USB
> connections and saves their info in the file
>      with open("EnumerateUSBLog.txt", "w") as wf:
>          counter = 0
>          for d in dev:
>              try:
>                  wf.write("USB Device number " + str(counter) + ":" + "\n")
>                  print("USB Device number " + str(counter) + ":" + "\n")
>                  wf.write(d._get_full_descriptor_str() + "\n")
>                  print(d._get_full_descriptor_str() + "\n")
> #                wf.write(d.get_active_configuration() + "\n")
>                  wf.write("\n")
>                  counter += 1
>              except NotImplementedError:
>                  wf.write("Device number " + str(counter) + "is busy." +
> "\n")
>                  wf.write("\n")
>                  counter += 1
>              except usb.core.USBError:
>                  wf.write("Device number " + str(counter) + " is either
> disconnected or not found." + "\n")
>                  wf.write("\n")
>                  counter += 1
>          wf.close()
>
> EnumerateUSB()
>