Stopping a Program

Question

Is there any way in Python to stop program execution in the middle of the code?

Answer

Many programs written by scientists are not complex enough to require the use of a debugger. Many of us prefer to check our code by putting in stop statements and printing out the values of selected variables.

Python does not have a stop statement. I looked really really hard for one. There is a break statement for getting out of loops, and there's an elegant exception handler, but nothing exists in the language to just stop program execution to let you inspect the state of the program.

Thankfully, through one of the methods in the Pdb debugger (included with the standard Python distribution), we can simulate a stop statement. If we are executing code in an interpreter window, the program will even stop execution and allow us to inspect any variable we want, like in IDL.

I learned how to do this from Stephen Berg's excellent article Debugging in Python. Here I'll summarize how to use Pdb to stop program execution.

Say we have the following simple program:

a = 1
b = a+2
print 'Line 3'
print 'Line 4'

and we wish to put a "stop" after b = a+2, like this:

a = 1
b = a+2
stop
print 'Line 3'
print 'Line 4'

To do this, I add the following snippet of code at the "stop":

import pdb #@@@
pdb.set_trace() #@@@
print 'stop here' #@@@


The @@@ comments are there so I can easily find those lines to remove them after I've finished debugging. So, the program now reads:

a = 1
b = a+2
import pdb #@@@
pdb.set_trace() #@@@
print 'stop here' #@@@
print 'Line 3'
print 'Line 4'

When you run the program, after b = a+2 executes, you will be put into the Pdb debugger. Your Python session (I'm assuming you're using the interactive interpreter) will look something like this:

>>> execfile('foo.py')
--Return--
> /usr/lib/python2.1/pdb.py(895)set_trace()->None
-> Pdb().set_trace()
(Pdb)


Pdb has this interesting feature in that when you first invoke it (via the pdb.set_trace() call), it enters the "Pdb" mode but doesn't do or know anything. For instance, if you try to inspect a program variable, it doesn't know the variable exists.

To see everything in your program, you have to step forward one line, by typing n at the (Pdb) prompt. You'll now get something like:

(Pdb) n
> /home/jlin/foo.py(5)?()
-> print 'stop here' #@@@
(Pdb)


At this point, you can print whatever is currently defined in your program. In fact, you can now execute any interactive Python statement (except definitions like loops, etc.) that you want at the (Pdb) prompt. There are some exceptions; sometimes, you have to add an exclamation point (!) to the statement in order for it to work. See Debugging in Python for details.

So, if you're interested in seeing the value of b, at the (Pdb) prompt type:

(Pdb) !b
3
(Pdb)


and the value of b is returned. Note that b without the ! means "set breakpoint" in Pdb. We add the ! so Pdb knows to return the value of variable b, not to set a breakpoint.

When you're done, press q to get out of Pdb. You'll get a bunch of traceback messages, before you return to the interactive Python prompt (>>>), but just ignore those messages.