fix thorlabs filter wheel position read error (#215)

* second call if to get position if first isn't int

* ignore empty outputs when waiting for reading

* Improve exception handling

Author: thomasmfish

microscope/filterwheels/thorlabs.py

@@ -18,6 +18,7 @@
 ## along with Microscope.  If not, see <http://www.gnu.org/licenses/>.
 
 import io
+import string
 import threading
 import warnings
 
@@ -80,29 +81,36 @@ def _do_set_position(self, new_position: int) -> None:
 
     def _do_get_position(self):
         # Thorlabs positions start at 1, hence the -1
-        return int(self._send_command("pos?")) - 1
+        try:
+            return int(self._send_command("pos?")) - 1
+        except TypeError:
+            raise microscope.DeviceError(
+                "Unable to get position of %s", self.__class__.__name__
+            )
 
     def _readline(self):
         """Custom _readline to overcome limitations of the serial implementation."""
-        result = [None]
+        result = []
         with self._lock:
-            while result[-1] not in ("\n", ""):
-                result.append(self.connection.read())
-        return "".join(result[1:])
+            while not result or result[-1] not in ("\n", ""):
+                char = self.connection.read()
+                # Do not allow lines to be empty.
+                if result or (char not in string.whitespace):
+                    result.append(char)
+        return "".join(result)
 
     def _send_command(self, command):
         """Send a command and return any result."""
-        result = None
         with self._lock:
             self.connection.write(command + self.eol)
             response = "dummy"
-            while response not in [command, ""]:
+            while command not in response and ">" not in response:
                 # Read until we receive the command echo.
-                response = self._readline().strip("> \n\r")
+                response = self._readline().strip()
             if command.endswith("?"):
                 # Last response was the command. Next is result.
-                result = self._readline().strip()
-        return result
+                return self._readline().strip()
+        return None
 
 
 class ThorlabsFW102C(ThorlabsFilterWheel):