I’m using pytest for my test suite. While catching bugs in complex inter-components test, I would like to place import ipdb; ipdb.set_trace() in the middle of my code to allow me to debug it.

However, since pytest traps sys.stdin/sys.stdout ipdb fails. How can I use ipdb while testing with pytest.

I’m not interested in jumping to pdb or ipdb after a failure, but to place breaks anywhere in the code and be able to debug it there before the failure occurs.

The error is raised because pytest captures output by default.

You can run pytest with -s option (turn off capture output). For example:

py.test -s my_test.py

and then in my_test.py:

import ipdb;
ipdb.set_trace()

pytest-ipdb is unfortunately not supported anymore.

The solution is to run
pytest my_test.py --pdb --pdbcls=IPython.terminal.debugger:Pdb

From the help command:

pytest -h
  --pdb                 start the interactive Python debugger on errors.
  --pdbcls=modulename:classname
                        start a custom interactive Python debugger on errors.
                        For example:
                        --pdbcls=IPython.terminal.debugger:TerminalPdb

The difference is just that TerminalPdb seems to throw erros, but Pdb not (Ipython docs).

As of 2019-11 here is what should fix it:

pip install ipdb gnureadline ptpython

export PYTEST_ADDOPTS='--pdb --pdbcls=IPython.terminal.debugger:Pdb'

This is what I use

py.test tests/ --pdbcls=IPython.core.debugger:Pdb -s

You may want to give pdbpp a try. I’ve had more success with it, compared to ipdb when used with pytest. See my answer here: https://stackoverflow.com/a/69320311/2896799.