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

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1765 TILs and counting..._
+_1766 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1060,6 +1060,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Look Inside Pytest tmp_path](python/look-inside-pytest-tmp-path.md)
 - [Override The Boolean Context Of A Class](python/override-the-boolean-context-of-a-class.md)
 - [Parse Relative Time To datetime Object](python/parse-relative-time-to-datetime-object.md)
+- [Start The Debugger When A Test Errors](python/start-the-debugger-when-a-test-errors.md)
 - [Store And Access Immutable Data In A Tuple](python/store-and-access-immutable-data-in-a-tuple.md)
 - [Test A Function With Pytest](python/test-a-function-with-pytest.md)
 - [Use pipx To Install End User Apps](python/use-pipx-to-install-end-user-apps.md)

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

@@ -0,0 +1,27 @@
+# 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.