Re: [pyusb-users] No Backend Available

[Steve K. <skr...@ve...>]

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 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.

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?

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()