How to handle test failures

Stopping after the first (or N) failures

To stop the testing process after the first (N) failures:

pytest -x           # stop after first failure
pytest --maxfail=2  # stop after two failures

Using PDB (Python Debugger) with pytest

Dropping to PDB (Python Debugger) on failures

Python comes with a builtin Python debugger called PDB. pytest allows one to drop into the PDB prompt via a command line option:

pytest --pdb

This will invoke the Python debugger on every failure (or KeyboardInterrupt). Often you might only want to do this for the first failing test to understand a certain failure situation:

pytest -x --pdb   # drop to PDB on first failure, then end test session
pytest --pdb --maxfail=3  # drop to PDB for first three failures

Note that on any failure the exception information is stored on sys.last_value, sys.last_type and sys.last_traceback. In interactive use, this allows one to drop into postmortem debugging with any debug tool. One can also manually access the exception information, for example:

>>> import sys
>>> sys.last_traceback.tb_lineno
42
>>> sys.last_value
AssertionError('assert result == "ok"',)

Dropping to PDB at the start of a test

pytest allows one to drop into the PDB prompt immediately at the start of each test via a command line option:

pytest --trace

This will invoke the Python debugger at the start of every test.

Setting breakpoints

To set a breakpoint in your code use the native Python import pdb;pdb.set_trace() call in your code and pytest automatically disables its output capture for that test:

  • Output capture in other tests is not affected.

  • Any prior test output that has already been captured and will be processed as such.

  • Output capture gets resumed when ending the debugger session (via the continue command).

Using the builtin breakpoint function

Python 3.7 introduces a builtin breakpoint() function. Pytest supports the use of breakpoint() with the following behaviours:

  • When breakpoint() is called and PYTHONBREAKPOINT is set to the default value, pytest will use the custom internal PDB trace UI instead of the system default Pdb.

  • When tests are complete, the system will default back to the system Pdb trace UI.

  • With --pdb passed to pytest, the custom internal Pdb trace UI is used with both breakpoint() and failed tests/unhandled exceptions.

  • --pdbcls can be used to specify a custom debugger class.

Fault Handler

New in version 5.0.

The faulthandler standard module can be used to dump Python tracebacks on a segfault or after a timeout.

The module is automatically enabled for pytest runs, unless the -p no:faulthandler is given on the command-line.

Also the faulthandler_timeout=X configuration option can be used to dump the traceback of all threads if a test takes longer than X seconds to finish (not available on Windows).

Note

This functionality has been integrated from the external pytest-faulthandler plugin, with two small differences:

  • To disable it, use -p no:faulthandler instead of --no-faulthandler: the former can be used with any plugin, so it saves one option.

  • The --faulthandler-timeout command-line option has become the faulthandler_timeout configuration option. It can still be configured from the command-line using -o faulthandler_timeout=X.

Warning about unraisable exceptions and unhandled thread exceptions

New in version 6.2.

Note

These features only work on Python>=3.8.

Unhandled exceptions are exceptions that are raised in a situation in which they cannot propagate to a caller. The most common case is an exception raised in a __del__ implementation.

Unhandled thread exceptions are exceptions raised in a Thread but not handled, causing the thread to terminate uncleanly.

Both types of exceptions are normally considered bugs, but may go unnoticed because they don’t cause the program itself to crash. Pytest detects these conditions and issues a warning that is visible in the test run summary.

The plugins are automatically enabled for pytest runs, unless the -p no:unraisableexception (for unraisable exceptions) and -p no:threadexception (for thread exceptions) options are given on the command-line.

The warnings may be silenced selectively using the pytest.mark.filterwarnings mark. The warning categories are pytest.PytestUnraisableExceptionWarning and pytest.PytestUnhandledThreadExceptionWarning.