Add Start The Debugger When A Test Errors as a Python TIL

python/start-the-debugger-when-a-test-errors.md

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+# Start The Debugger When A Test Errors
+
+While working on [some
+tests](https://github.com/jbranchaud/build-an-llm-from-scratch/blob/main/tests/chapter_02/test_bpe_tokenizer.py)
+for my Byte Pair Encoding tokenizer, I was running into an unexpected test
+failure. To better understand what was going on, I needed to inspect the state
+of the program around the time the code raised an exception.
+
+Instead of needing to manually set a breakpoint at the correct spot to begin
+debugging, I can run the test with the Pytest-supported `--pdb` flag. That's
+short for _python debugger_.
+
+> Start the interactive Python debugger on errors or KeyboardInterrupt
+
+What this does during a test run is opens you up to the interactive Python
+debugger at the exact moment an exception is raised. This gives you the ability
+to inspect values of the program state at that point in execution which could
+help inform the needed fix.
+
+```bash
+uv run pytest -vv --pdb -k "test_train_bpe"
+```
+
+There I am running a specific test that matches against `-k "test_train_bpe"`
+and the python debugger will start up if there is an error.
+
+See `uv run pytest --help` for more details.