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

Answer 1

The following script will run a module and break into post-mortem debugging if an exception occurs while running the module. It should work both with Python 2.7 and 3.x.

Usage:

mdb.py module_name [args ...]

Known limitations:

• While running the module code, sys.argv[0] is preserved as the module name, instead of being resolved to the file path of the module.
• If the target module is not found, the error is not reported any differently than if the error occurred during the execution of the module

---

mdb.py

#!/usr/bin/env python

from __future__ import print_function
import pdb
import runpy
import sys
import traceback

if len(sys.argv) == 0:
    print("Usage: mdb.py module_name [args ...]")
    exit(1)

modulename = sys.argv[1]
del sys.argv[0]

try:
    runpy.run_module(modulename, run_name='__main__')
except:
    traceback.print_exception(*sys.exc_info())
    print("")
    print("-" * 40)
    print("mdb: An exception occurred while executing module ", modulename)
    print("mdb: See the traceback above.")
    print("mdb: Entering post-mortem debugging.")
    print("-" * 40)
    pdb.post_mortem(sys.exc_info()[2])

---

Demonstration:

$ tree
.
├── mdb.py
└── mypackage
    ├── __init__.py
    ├── __main__.py
    └── mymodule.py

1 directory, 4 files

$ ###################### Examine the module code ###################
$ cat mypackage/mymodule.py 
from __future__ import print_function
import sys

print("mymodule loaded")

if __name__ == "__main__":
    print("mymodule executed")
    print("args:", sys.argv)

$ #################### Run the module through python ###############
$ python -m mypackage.mymodule abc defgh
mymodule loaded
mymodule executed
args: ['/home/leon/playground/mdb/mypackage/mymodule.py', 'abc', 'defgh']

$ #################### Run the module through mdb ##################
$ ./mdb.py mypackage.mymodule abc defgh
mymodule loaded
mymodule executed
args: ['mypackage.mymodule', 'abc', 'defgh']
$ ###   ^^^^^^^^^^^^^^^^^^
$ ### Note that sys.argv[0] is not resolved to the file path

$ ###################### Examine the module code ###################
$ cat mypackage/__main__.py 
from __future__ import print_function
import sys

print("mypackage loaded")

if __name__ == "__main__":
    print("mypackage executed")
    print("args:", sys.argv)
    print(x + y)

$ #################### Run the module through python ###############
$ python -m mypackage
mypackage loaded
mypackage executed
args: ['/home/leon/playground/mdb/mypackage/__main__.py']
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/leon/playground/mdb/mypackage/__main__.py", line 9, in 
    print(x + y)
NameError: name 'x' is not defined

$ #################### Run the module through mdb ##################
$ ./mdb.py mypackage
mypackage loaded
mypackage executed
args: ['mypackage']
Traceback (most recent call last):
  File "./mdb.py", line 17, in 
    runpy.run_module(modulename, run_name='__main__')
  File "/usr/lib/python2.7/runpy.py", line 192, in run_module
    fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/leon/playground/mdb/mypackage/__main__.py", line 9, in 
    print(x + y)
NameError: name 'x' is not defined

----------------------------------------
mdb: An exception occurred while executing module  mypackage
mdb: See the traceback above.
mdb: Entering post-mortem debugging.
----------------------------------------
> /home/leon/playground/mdb/mypackage/__main__.py(9)()
-> print(x + y)
(Pdb) q