Fix fileno error on Windows (robotell bus) (#1313)

* Fix fileno error on Windows (robotell bus)

* Fix format

* Add test

* format

Author: gribera

can/interfaces/robotell.py

@@ -2,6 +2,7 @@
 Interface for Chinese Robotell compatible interfaces (win32/linux).
 """
 
+import io
 import time
 import logging
 
@@ -367,10 +368,14 @@ def shutdown(self):
         self.serialPortOrig.close()
 
     def fileno(self):
-        if hasattr(self.serialPortOrig, "fileno"):
+        try:
             return self.serialPortOrig.fileno()
-        # Return an invalid file descriptor on Windows
-        return -1
+        except io.UnsupportedOperation:
+            raise NotImplementedError(
+                "fileno is not implemented using current CAN bus on this platform"
+            )
+        except Exception as exception:
+            raise CanOperationError("Cannot fetch fileno") from exception
 
     def get_serial_number(self, timeout):
         """Get serial number of the slcan interface.

test/test_robotell.py

@@ -940,6 +940,10 @@ def test_set_hw_filter(self):
             ),
         )
 
+    def test_when_no_fileno(self):
+        with self.assertRaises(NotImplementedError):
+            self.bus.fileno()
+
 
 if __name__ == "__main__":
     unittest.main()