Refactor assertions that use walrus (#18496)

src/lightning/pytorch/CHANGELOG.md

@@ -35,6 +35,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed the display of `v_num` in the progress bar when running with `Trainer(fast_dev_run=True)` ([#18491](https://github.com/Lightning-AI/lightning/pull/18491))
 
 
+- Fixed `UnboundLocalError` when running with `python -O` ([#18496](https://github.com/Lightning-AI/lightning/pull/18496))
+
 
 - Fixed visual glitch with the TQDM progress bar leaving the validation bar incomplete before switching back to the training display ([#18503](https://github.com/Lightning-AI/lightning/pull/18503))
 

src/lightning/pytorch/loops/fetchers.py

@@ -52,10 +52,10 @@ def __iter__(self) -> "_DataFetcher":
         return self
 
     def __next__(self) -> Any:
-        assert (iterator := self.iterator) is not None
+        assert self.iterator is not None
         self._start_profiler()
         try:
-            batch = next(iterator)
+            batch = next(self.iterator)
         except StopIteration:
             self.done = True
             raise

tests/tests_pytorch/run_standalone_tasks.sh

@@ -34,3 +34,7 @@ MASTER_ADDR="localhost" MASTER_PORT=1234 LOCAL_RANK=0 python strategies/scripts/
 # test that ddp can launched as a module (-m option)
 echo "Running ddp example as module"
 python -m strategies.scripts.cli_script ${args}
+
+# test that `python -O` (optimized mode) works
+echo "Running bug report model with 'python -O'"
+python -O ../../examples/pytorch/bug_report/bug_report_model.py