Resolve an unset data_bitrate to no operation in slcan. (#1978)

* Change the default to 0, which is a no-op for data_bitrate.

* Added a changelog fragment.

* Fix black formatting for test_slcan.py

* Revert the function signature, and explicitly check for `None` in data_bitrate.

* Update the town crier news fragment.

Author: dcasnowdon

can/interfaces/slcan.py

@@ -180,6 +180,11 @@ def set_bitrate(self, bitrate: int, data_bitrate: Optional[int] = None) -> None:
         else:
             bitrates = ", ".join(str(k) for k in self._BITRATES.keys())
             raise ValueError(f"Invalid bitrate, choose one of {bitrates}.")
+
+        # If data_bitrate is None, we set it to 0 which means no data bitrate
+        if data_bitrate is None:
+            data_bitrate = 0
+
         if data_bitrate in self._DATA_BITRATES:
             dbitrate_code = self._DATA_BITRATES[data_bitrate]
         else:

doc/changelog.d/1978.fixed.md

@@ -0,0 +1 @@
+Fix initialisation of an slcan bus, when setting a bitrate. When using CAN 2.0 (not FD), the default setting for `data_bitrate` was invalid, causing an exception.

test/test_slcan.py

@@ -74,7 +74,13 @@ class slcanTestCase(unittest.TestCase):
     def setUp(self):
         self.bus = cast(
             can.interfaces.slcan.slcanBus,
-            can.Bus("loop://", interface="slcan", sleep_after_open=0, timeout=TIMEOUT),
+            can.Bus(
+                "loop://",
+                interface="slcan",
+                sleep_after_open=0,
+                timeout=TIMEOUT,
+                bitrate=500000,
+            ),
         )
         self.serial = cast(SerialMock, self.bus.serialPortOrig)
         self.serial.reset_input_buffer()