Forcing a stop is useful when your program locks up and won’t respond. The trick is to press Ctrl+C (the Ctrl key and the C key at the same time; don ‘ t press the Shift key). Make sure the Python window is active (by clicking the window) when you do — or you might close the wrong 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 torequire the use of a debugger. Many of us prefer to check ourcode by putting in stop statements and printingout the values of selected variables.

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

Thankfully, through one of the methods in the Pdbdebugger (included with the standard Python distribution),we can simulate a stop statement. Ifwe are executing code in an interpreter window, the programwill even stop execution and allow us to inspect any variablewe want, like in IDL.

I learned how to do this from Stephen Berg's excellent articleDebugging in Python. Here I'll summarize how touse 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:

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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 thoselines 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+2executes, you will be put into the Pdb debugger. YourPython session (I'm assuming you're using the interactiveinterpreter) 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 tryto inspect a program variable, it doesn't know the variableexists.

To see everything in your program, you have to step forward oneline, 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 inyour program. In fact, you can now execute any interactivePython statement (except definitions like loops, etc.)that you want at the (Pdb) prompt. There aresome exceptions; sometimes, you have to add an exclamation point(!) to the statement in order for it to work. SeeDebugging 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 bwithout 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 ofPdb. You'll get a bunch of traceback messages, before youreturn to the interactive Python prompt (>>>), but just ignore those messages.

Is this hard?

I am introducing Python to my students and want to make a simple, interactive program that will, while it's running, 'listen' for any keyboard events. That is, I don't want to include an 'input' statement - that would pause execution till someone answered. I just want it to keep running, say, printing an 'a' on the screen every 10 seconds, until the user types a 'b'. Then printing a 'b' every 10 seconds till the user types in some other character..

It seems that this should not be hard; something with sys.stdin, perhaps. The hours I've spent on it are getting embarrassing.

I've fooled around with it and looked though books and the only solution I've come up with was to import pygame.py and use its event handling. This seems like massive overkill for what should be an easy matter.

I'm using IDLE with MacPython 2.5.1 on Mac OS 10.4.11

Thanks for any help on this problem.

Neal C.

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