Pytest excels at helping you with test failures.
After running your tests via py.test, you will see an error report with a
detailed traceback for each of the failures, if any. You can change the mode
in which output is presented to you via the
--tb cli option:
py.test --tb=auto # (default) 'long' tracebacks for the first and last # entry, but 'short' style for the other entries py.test --tb=long # exhaustive, informative traceback formatting py.test --tb=short # shorter traceback format py.test --tb=line # only one line per failure py.test --tb=native # Python standard library formatting py.test --tb=no # no traceback at all
Have you ever heard of pdb - the Python Debugger?
I strongly recommend checking it out, if you haven’t used it yet. The standard library’s built-in debugger will prove useful for debugging your Python code without installing any external dependencies or setting up some fancy IDE.
If invoked with the
--pdb option, pytest will place a debugger breakpoint
whenever an error occurs in your tests.
$ py.test --pdb
You can also set a debugger breakpoint yourself with
import pdb; pdb.set_trace().
pp to inspect the test code.
If you want to get a list of all local variables and their value, it’s best to
run pytest with the
-l option enabled, as
locals() may contain internal
pytest items if you print it from the breakpoint.
Useful CLI flags
Run only broken tests
An underrated feature of pytest is the
--lf option, which tells pytest to
only run the tests that failed the last time it was executed.
Once you’ve encountered any errors in your tests, you want to focus on the failures and get a better understanding of what’s causing the problems as opposed to spending time on running tests that are perfectly fine.
Re-order tests - failures first
Keep in mind though, whenever you modify your code other tests may break.
If you run
$ py.test --ff all tests will be executed, but re-ordered based
on whether they’ve failed in the previous run or not. Failures first,
successful tests after.
Stop suite after n failures
You can end the test session early by pressing
CTRL-C. Although pytest
KeyboardInterrupt error and runs the teardown code before
closing the session gracefully, it is definitely not the best way to stop the
-x to stop after the first failure, or
--maxfail=N to stop
after the first N failures.