========================= Avoiding debugging issues ========================= When you set a breakpoint, e.g. via ``import pdb;pdb.set_trace()`` and run a collection of tests, you may receive the following message: .. code-block:: shell-session Can't use pdb.set_trace when running a layer as a subprocess! This happens when you are running a collection of tests which span multiple layers, and a previous layer could not be safely torn down in-process, so the testrunner had to spawn a subprocess instead. The workaround is to run just the one test you are trying to debug. You can read more about this issue in `zope.testrunner's documentation `_.