debugging: how to check what where my Python program is hanging?

Question

A fairly large Python program I write, runs, but sometimes, after running for minutes or hours, in a non easily reproducible moment, hangs and outputs nothing to the screen.

I have no idea what it is doing at that moment, and in what part of code it is.

How can I run this in a debugger or something to see what lines of code is the program executing in the moment it hangs?

Its too large to put "print" statements all over the place.

I did:

python -m trace --trace /usr/local/bin/my_program.py

but that gives me so much output that I can't really see anything, just millions of lines scrolling on the screen.

Best would be if I could send some signal to the program with "kill -SIGUSR1" or something, and at that moment the program would drop into a debugger and show me the line it stopped at and possibly allow me to step through the program then.

I've tried:

pdb usr/local/bin/my_program.py

and then:

(Pdb) cont

but what do I do to see where I am when it hangs? It doesn't throw and exception, just seems like it waits for something, possibly in an infinite loop.

One more detail: when the program hangs, and I press ^C and then (not sure if that is necessary) the program continues normally (without throwing any exception and without giving me any hint on the screen why did it stop).

Answer 1

This could be useful to you. I usually do

>>> import pdb
>>> import program2debug
>>> pdb.run('program2debug.test()')

I usually add a -v option to my programs, which enables tons of print statements explaining what I'm doing in detail. When you write a program in the future, consider doing the same before it gets thousands of lines big.