How to debug a Python module run with python -m from the command line?

Question

I know that a Python script can be debugged from the command line with

python -m pdb my_script.py

if my_script.py is a script intended to be run with python my_script.py.

However, a python module my_module.py should be run with python -m my_module. Even scripts that contain relative imports should be run with python -m. How can I run python -m my_module under pdb's control? The following does not work:

python -m pdb -m my_module

Answer 1

You can't do it now, because -m terminates option list

python -h
...
-m mod : run library module as a script (terminates option list)
...

That means it's mod's job to interpret the rest of the arguments list and this behavior fully depends on how mod is designed internally and whether it support another -m

Lets check out what's happening inside pdb of python 2.x. Actually, nothing intereseting, it only expects a script name to be supplied:

   if not sys.argv[1:] or sys.argv[1] in ("--help", "-h"):
        print "usage: pdb.py scriptfile [arg] ..."
        sys.exit(2)

    mainpyfile =  sys.argv[1]     # Get script filename
    if not os.path.exists(mainpyfile):
        print 'Error:', mainpyfile, 'does not exist'
        sys.exit(1)

    del sys.argv[0]         # Hide "pdb.py" from argument list

    # Replace pdb's dir with script's dir in front of module search path.
    sys.path[0] = os.path.dirname(mainpyfile)

    # Note on saving/restoring sys.argv: it's a good idea when sys.argv was
    # modified by the script being debugged. It's a bad idea when it was
    # changed by the user from the command line. There is a "restart" command
    # which allows explicit specification of command line arguments.
    pdb = Pdb()
    while True:
        try:
            pdb._runscript(mainpyfile)

Same for the currently released versions of python 3.x

Good news

The pull request that allows to do what you're asking has been merged 5 days ago. What a mysterious coincidence! Here's the code

So just wait a bit for the upcoming python 3.x versions to have this issue resolved )