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(i)PDB post mortem when using 'unittest discover'

Most of the times when I run my tests, I do this via python -m unittest discover (I'm lazy).
Lets say that one of the tests raises an exception. Is there a way to make unittest framework run a post-mortem on it (preferably ipdb.pm()), without modifying the code of the tests?
I know I could add it directly to the code, but since I also use automatic runners / GitlabCI, I don't want those to hang up on the pdb shell.

python: pytest for cmd tools

I created a Python library and a set of Python script around it. An example of this small script could be something like this rna_ex2x.py:
./rna_ec2x.py
usage: rna_ec2x.py [-h] [--sep SEP] [--chain CHAIN] [--ec-pairs]
[--ss-pairs SS_PAIRS] [--pairs-delta]
interaction_fn
rna_ec2x.py: error: too few arguments
I want to test these script with pytest. I know how to test my functions with pytest, but I can't find in the documentation what would be the best practice in testing standalone Python script. Any suggestions?

I don't know what would be the best practice but I simply call my programs using subprocess.call(), check the result code and verify that the program did what it intended to do. See my tests as examples.

pytest run only the changed file?

I'm fairly new to Python, trying to learn the toolsets.
I've figured out how to get py.test -f to watch my tests as I code. One thing I haven't been able to figure out is if there's a way to do a smarter watcher, that works like Ruby's Guard library.
Using guard + minitest the behavior I get is if I save a file like my_class.rb then my_class_test.rb is executed, and if I hit enter in the cli it runs all tests.
With pytest so far I haven't been able to figure out a way to only run the test file corresponding to the last touched file, thus avoiding the wait for the entire test suite to run until I've got the current file passing.
How would you pythonistas go about that?
Thanks!

One possibility is using pytest-testmon together with pytest-watch.
It uses coverage.py to track which test touches which lines of code, and as soon as you change a line of code, it re-runs all tests which execute that line in some way.

To add to #The Compiler's answer above, you can get pytest-testmon and pytest-watch to play together by using pytest-watch's --runner option:
ptw --runner "pytest --testmon"
Or simply:
ptw -- --testmon

There is also pytest-xdist which has a feature called:
--looponfail: run your tests repeatedly in a subprocess. After each run py.test waits until a file in your project changes and then re-runs the previously failing tests. This is repeated until all tests pass after which again a full run is performed.

The fastest setup I got was when I combines #lmiguelvargasf #BenR and #TheCompiler answer into this
ptw --runner "pytest --picked --testmon"
you first gotta have them installed by
pip3 install pytest-picked pytest-testmon pytest-watch

If you are using git as version control, you could consider using pytest-picked. This is a plugin that according to the docs:
Run the tests related to the unstaged files or the current branch
Demo
Basic features
Run only tests from modified test files
Run tests from modified test files first, followed by all unmodified tests
Usage
pytest --picked

Pydev run unittests selectively

When I use Java with JUnit in Eclipse, I can highlight a method name in a unit test file and run only that single test. Is something like this available in PyDev? I don't want to run all my tests in a file all the time, only a single one.

In PyDev you must do Ctrl+F9 and then select the test to run.
Also, it may be nice reading: http://pydev.org/manual_101_run.html as it gives some more hints on running modules within PyDev.

How to use `pytest` from Python?

I'm working in a project that recently switched to the pytest unittest framework. I was used to calling my tests from Eclipse, so that I can use the debugger (e.g. placing breakpoints to analyze how a test failure develops). Now this is no longer possible, since the only way to run the tests is via the command line blackbox.
Is there some way to use pytest from within Python, so that one is not forced to drop out of the IDE? The tests should of course not be run in a separate process.

I think I can now answer my own question, it's pretty simple:
import pytest
pytest.main(args)
which is documented in the Section "Calling pytest from Python code".
Then I can run this module and/or start it with the integrated debugger.
args is the list of command-line arguments, so for example to run only particular tests I can use something like:
args_str = "-k test_myfavorite"
args = args_str.split(" ")
pytest.main(args)

It seems that now (py.test version 2.0+) someone can also do this :
import pytest
pytest.main('-x {0}'.format(argument))
# Or
# pytest.main(['-x', 'argument'])
Ref

This is now supported by pytest and described nicely in the documentation.
You can invoke pytest from Python code directly:
import pytest
pytest.main()
this acts as if you would call “pytest” from the command line. It will not raise SystemExit but return the exitcode instead. You can pass in options and arguments:
pytest.main(["-x", "mytestdir"])

For me it was this:
pytest.main(["-x", "path to test file", "args"])
For example:
import pytest
pytest.main(["-x", "/api/test", "-vv"])

Maybe you could give a try to pycharm it has direct integration with py.test (I use it at work) and debugger runs perfectly.

I have not tried with eclipse, but as was suggested in a related question, it is possible to use the --pdb command line option with py.test. Maybe it is possible to configure eclipse that way.
However, calling the standard import pdb;pdb.set_trace() will not directly call the debugger. First it will issue an error which in turn will activate the debugger. This might or might not make things work differently.

You can just run py.test --pdb if you just want to a debugger and don't need the IDE