Created attachment 1647536 [details]
Example fix

Description of problem:
Calibre companion app cannot connect - Python TypeError in smart device driver.


Version-Release number of selected component (if applicable): 4.6.0-1.fc31


How reproducible:
1. Start wireless device server
2. Connect device


Actual results:
Device disconnects without error displayed in either GUI or device.
Traceback visible if calibre launched from terminal.

Traceback (most recent call last):
  File "/usr/lib64/calibre/calibre/devices/smart_device_app/driver.py", line 638, in _call_client
    self._send_byte_string(self.device_socket, (b'%d' % len(s)) + s)
TypeError: can't concat str to bytes
Traceback (most recent call last):
  File "/usr/lib64/calibre/calibre/gui2/device.py", line 298, in detect_device
    dev.open(cd, self.current_library_uuid)
  File "/usr/lib64/calibre/calibre/devices/smart_device_app/driver.py", line 49, in _synchronizer
    return func(self, *args, **kwargs)
  File "/usr/lib64/calibre/calibre/devices/smart_device_app/driver.py", line 1050, in open
    'canSupportLpathChanges': True})
  File "/usr/lib64/calibre/calibre/devices/smart_device_app/driver.py", line 49, in _synchronizer
    return func(self, *args, **kwargs)
  File "/usr/lib64/calibre/calibre/devices/smart_device_app/driver.py", line 638, in _call_client
    self._send_byte_string(self.device_socket, (b'%d' % len(s)) + s)
TypeError: can't concat str to bytes


Expected results:
Device connects and syncs to Calibre.

Additional info:

Explicitly encoding string to UTF-8 works - patch attached.
Not sure whether there is a better option for fixing this, but it got my device  synchronising again.