TweetFollow Us on Twitter

Mac in the Shell: Debugging Python

Volume Number: 25
Issue Number: 11
Column Tag: Mac in the Shell

Mac in the Shell: Debugging Python

Stepping through code with pdb

by Edward Marczak

Welcome

We've been covering Python in this column for the last few months. We've gone from the basics, such as built in data types, variable assignment and so on, through more advanced concepts such as creating classes and integrating with Cocoa via PyObj-C. The intent was never to imagine that this column alone would turn you into a master Python programmer, but to give you the tools and direction to do so. One tool that you will need on that journey, though, is a decent debugger. While it's less common in scripting languages like Python and Ruby to use a debugger, when something is just not working out as expected and you can't figure out why, a peek at the code while it's running is invaluable. This month, I'll show you how to do that in Python using the python debugger ("pdb").

Do The Needful

Using a Shell

The instructions in this column always try to respect the way people are used to working. However, debuggers are interactive and grew up in a shell environment. While there may be interaction with certain editors, that will be outside of the scope for this article. Edit in whatever editor you like, but we're going to run and debug from a shell. (I think the general unease with the shell is lessening in the Mac community...right?). So, fire up Terminal.app (or iTerm, Terminator, etc.) and we'll get started.

Learning Your History

A debugger is itself a program that lets you examine another running program. You can use a debugger to step through the running code of the target program one line at a time, examine the values of variables at a given point in the code, run up until a certain breakpoint and examine a program crash or exception. One of the more well-known multi-language debuggers is the GNU Debugger, or "gdb." While you could use gdb to debug Python, there happens to exist a Python-specific clone of GDB called pdb, or, the Python Debugger.

In many scripting languages, programs are typically relatively short, and debuggers are often unnecessary. Many scripters are accustomed to sprinkling 'print' or logging statements through their code that reveals the value of particular variables at a particular point in the program's execution. However, you may have realized that Python is a bit more grown up than many traditional scripting languages. There are many fairly large systems written in Python. As an application gets larger and contains more dependencies, a dedicated debugger becomes not only useful, but necessary.

Getting Started

We'll start off with some simple code as an example of basic debugging. You write the code in listing 1 in the hopes finding prime numbers through and including 10.

Listing 1: prime_debug.py - sample code for debugging.

#!/usr/bin/python
for n in range(2, 10):
  for x in range(2, n):
    if n % x == 0:
      print n, 'equals', x, '*', n/x
      break
    else:
      print n, 'is a prime number'

Of course, you run this code and see something a little different than you expected—there are two problems in this code. A basic reason for debugging! (Kudos if you already see the errors).

The Python debugger is implemented as a module, so, like other modules, you need to import it. Add the following import after the shebang line:

import pdb

You'll also need to pick a point where you want to start tracing. Since this is an example, we'll start right at the top. So also add the set_trace method immediately following the import statement:

pdb.set_trace()

Now you can just run the program (mark it as executable first with a chmod 770 prime_debug.py or simlar). However, when you run the program this time, you're looking at something different. Something like this:

$ ./prime_debug.py > /Users/marczak/dev/py/prime_debug.py(7)<module>() -> for n in range(2, 10): (Pdb)

What you are looking at is the pdb interactive debugger waiting for your command. You'll see this when the pdb.set_trace() method is called. At this point, pdb stops all execution, displays the statement that it's waiting to execute next and displays its prompt. For our purposes, we want to execute this line (for n in range(2, 10)), so, we enter n, for "next." After pressing return, we're greeted with new information and a new prompt:

-> for x in range(2, n):
(Pdb)

Ah! We've moved on to the next line of the program, and are looking at the next statement to execute. To do so, you can simply press return, as pdb will repeat the last command you gave it by pressing return. Keep doing this a few times until you're comfortable with the display and what you're looking at.

Just so we can get back in sync, quit the debugger and we'll start again. To quit pdb at any time, issue a q command. You'll see a diagnostic "bdb.BdbQuit" line printed and find yourself back at a shell prompt.

Run your program again and let it drop into the debugger, and let's do something a little more useful this time. Tracing program flow is useful, but just as useful is being able to examine the value of variables. You're now essentially waiting for the first line of the program to execute: "for n in range(2, 10)." If you try to examine the variable n right now, you'll receive an error, since this line hasn't yet executed and n isn't yet defined.

First, execute this first line by entering n for "next," then enter p n, which stands for "print the contents of n." You can display the contents of any variable with the p ("print") command. In our example, the output should look like this:

-> for x in range(2, n):
(Pdb) p n
2

This is completely in line with our expectations: n is 2, right at the beginning of its range. (Note that the displayed line is the next line, not the one we're examining the variable of).

Finding our problem

Let's go off and find our problem, which is actually two-fold. The output currently looks like this:

3 is a prime number
4 equals 2 * 2
5 is a prime number
5 is a prime number
5 is a prime number
6 equals 2 * 3
7 is a prime number
7 is a prime number
7 is a prime number
7 is a prime number
7 is a prime number
8 equals 2 * 4
9 is a prime number
9 equals 3 * 3

This is all technically correct, but ugly. What's with the repeating lines? Also, we wanted to find values through 10, not 9. Since the first time we see the repeating lines is when n is equal to 5, let's find that point. Run the program, step through each line using the n command until you see the first output of "5 is a prime number." It will look like this:

> /Users/marczak/dev/py/prime_debug.py (9)<module>()
-> if n % x == 0:
(Pdb) 
> /Users/marczak/dev/py/prime_debug.py (14)<module>()
-> print n, 'is a prime number'
(Pdb) 
5 is a prime number
> /Users/marczak/dev/py/prime_debug.py (8)<module>()
-> for x in range(2, n):
(Pdb) 

Now, let's pay attention as we continue to step through. After a few iterations (or sooner), it should become clear: our if statement is not True, which is fine, and the else clause is running our print statement, which isn't fine. We really only want to print that notification on the way out of the loop when it fails to find a factor. So, our logic error is simple: we have the wrong level of indentation on the else statement. It should be un-indented one level, to be a part of the for loop. The entire loop should look like this:

for n in range(2, 10):
  for x in range(2, n):
    if n % x == 0:
      print n, 'equals', x, '*', n/x
      break
  else:
    print n, 'is a prime number'

Again, notice the subtle difference in indentation for the else portion – it's really a part of the for loop. If you're 'too close' to your code, that's an easy one to miss. However, debugging can be similar to explaining your code to a rubber duck—you know how it's supposed to work, but you only have the 'a ha!' as you step through it.

Make it Easier

We found our major error, but now have another: we want to print primes up through and including the number 10. If you're like me, you need a refresher at this point as to where you are in the code. Issuing an l (not "one," but "ell," for "list") will do just that:

(Pdb) l
  3     import pdb
  4     
  5     pdb.set_trace()
  6     
  7     for n in range(2, 10):
  8  ->     for x in range(2, n):
  9         if n % x == 0:
 10           print n, 'equals', x, '*', n/x
 11           break
 12         else:
 13           # loop fell through without finding a factor

Ah! Now I know where I am. All we're really interested in from this point on is the value of n. Stepping through the remainder of the code shows that the initial for loop exits after 9. Didn't we ask it to run until 10?

Yes we did, but that's our misconception. Looking at the Python documentation for range() shows that the range intentionally excludes the final number.

While this may not be a common mistake that you make, it turns out that this is still a useful exercise: you may not always be debugging your own code.

Dealing with Functions

There's a few more pdb commands to understand before you tackle larger python programs. Specifically, you'll want to know how to deal with functions. Take, for example, the code in listing 2.

Listing 2: dict_iterate.py

#!/usr/bin/python
import pdb
pdb.set_trace()
def _PrintDict(dict):
  """Recursively iterate over a dictionary, printing results
  Args:
    dict: The dictionary to print
  """
  pdb.set_trace()
  for item in dict:
    if type(dict[item]) == dict:
      _PrintDict(dict[item])
    else:
      print "%s: %s" % (dict[item], type(dict[item]))
def main():
  """Main routine"""
  aDict = {'color': 'blue',
           'count': 15,
           'cust_info': {'pid': '94758476', 'uid': '348576'},
           'style': 'fruit'}
  _PrintDict(aDict)
if __name__ == "__main__":
  main()

This should look vaguely familiar to anyone who read the previous column on Python. Start this program running and step through it with n—you'll see python touch each function name to create an object for it. If you keep tracing with n ("next"), this program will end very quickly. This is because when the n command reaches a function, it executes the entire function without entering that function. So, stop tracing with n when you arrive at the call to main():

-> if __name__ == "__main__":
(Pdb) n
> /Users/marczak/dev/py/dict_iterate.py(30)<module>()
-> main()

We want to step into main(), so go ahead and enter s (for "step"). You should be greeted with:

def main():

showing that you're now looking at the definition for main(). Keep stepping as we want to also step into the call to _PrintDict().

When you do arrive in the _PrintDict() function, there's a for loop. Once you've traversed that loop, you may no longer be deeply interested in it, but want to get back to where you were before entering this function. pdb has a solution for you: r, for "return." Essentially, "finish up this function and return."

Be aware! Stepping into functions sometimes will have an unintended consequence for you: stepping into an library that you've included. This is often not the code that you're interested in debugging, though it may be. If you accidentally step into a library function—PyObj-C code included—just remember the r command and return until you're back to where you expect.

More pdb Features

You now know the core of pdb and can actually do some serious debugging. However, pdb offers a lot more. Some of which we'll save for another column, but there are two more useful things to pass on.

The easy way out: c, for "continue." If at any point, you've traced through all you've wanted to trace, but don't want to crash the program with a quit (q) command, there is another option. The continue command picks up and runs the remainder of the program.

Even better, though, is this: pdb is letting you load and run your Python program in an interactive environment. You can alter variables just by assigning them:

(Pdb) x =7
(Pdb) p x
7

Imagine the simple code in listing 3.

Listing 3: math.py

#!/usr/bin/python
import pdb
pdb.set_trace()
x = 5
for i in range(1, 10):
  print i + x

At any time after x gets assigned, you can reassign it. Your debugging session can look like this:

$ ./math.py 
> /Users/marczak/dev/py /math.py(7)<module>()
-> x = 5
(Pdb) n
> /Users/marczak/dev/py /math.py(9)<module>()
-> for i in range(1, 10):
(Pdb) 
> /Users/marczak/dev/py /math.py(10)<module>()
-> print i + x
(Pdb) 
6
> /Users/marczak/dev/py /math.py(9)<module>()
-> for i in range(1, 10):
(Pdb) x = 20
(Pdb) n
> /Users/marczak/dev/py /math.py(10)<module>()
-> print i + x
(Pdb) 
22

This is fantastic news if you want to test your code for fragility around scenarios where variable reach certain values. Of course, as the code runs, if a variable gets reassigned in your program, you need to watch for that.

Reference Sheet

Here's a handy list of the pdb topics discussed in this article:

pdb library: import pdb
Start tracing: pdb.set_trace()
n: execute next command
s: step into a function
r: return from function
c: continue running program
q: quit pdb (and error out of program).

Conclusion

While you may focus on shorted programs now, as your skills improve, your programs should grow in complexity and size. At some point, you'll likely confound yourself, and a debugger comes in very handy during these times. Fortunately, Python has an available debugger that is friendly to use and easy to make available to your program.

Media of the month: I've been in a "back to school" kind of way, but really just studying on my own. There's too much to learn, right? Well, I've taken the school approach: one topic a day and rotate through them and study each night. Now, I'm not recommending anything this drastic, but, I'd bet there's one subject that you've wanted to learn. Now is the time. Hit up your local bookstore, University or Amazon.com, find a book and go. If you're still an actual student, well, keep going!

Until next month, keep scripting.


Ed Marczak is the Executive Editor for MacTech Magazine, and has written the Mac in the Shell column since 2004.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Chromium 45.0.2454.85 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 45.0.2454.85: Note: Does not contain the "... Read more
OmniFocus 2.2.5 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
iFFmpeg 5.7.1 - Convert multimedia files...
iFFmpeg is a graphical front-end for FFmpeg, a command-line tool used to convert multimedia files between formats. The command line instructions can be very hard to master/understand, so iFFmpeg does... Read more
VOX 2.6 - Music player that supports man...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more
Box Sync 4.0.6567 - Online synchronizati...
Box Sync gives you a hard-drive in the Cloud for online storage. Note: You must first sign up to use Box. What if the files you need are on your laptop -- but you're on the road with your iPhone? No... Read more
Carbon Copy Cloner 4.1.4 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
OmniGraffle Pro 6.3.1 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
Monosnap 3.1.2 - Versatile screenshot ut...
Monosnap lets you capture screenshots, share files, and record video and .gifs! Capture: Capture full screen, just part of the screen, or a selected window Make your crop area pixel perfect with... Read more
Alfred 2.7.2 - Quick launcher for apps a...
Alfred is an award-winning productivity application for OS X. Alfred saves you time when you search for files online or on your Mac. Be more productive with hotkeys, keywords, and file actions at... Read more
Microsoft Remote Desktop 8.0.19 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more

We're Sorry to Report that Moonrise...
Moonrise is a very promising-looking, Pokemon-esque monster collecting and battling game that we were really looking forward to reviewing, but unfortunately it looks like that's never going to happen. [Read more] | Read more »
The Latest Update for The Sims FreePlay...
Commerce has gotten a little more active with the newest update for The Sims FreePlay, making Sunset Mall more of a hangout than ever before. [Read more] | Read more »
This Week at 148Apps: August 24-28, 2015
The Apps of August With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out... | Read more »
NASCAR in Real Racing 3? Sure, Why Not?
I have to give Firemonkeys credit - it's very cool of them to add NASCAR to Real Racing 3 via an update rather than making a separate game for it. But that's a different discussion for another time; for now let's sit back and enjoy driving in... | Read more »
The nuyu is an Inexpensive Activity Moni...
Today, Health o Meter nuyu has announced a series of health and fitness-related products, including the aforementioned activity monitor along with a wireless scale. All at a decent pricepoint, no less. [Read more] | Read more »
The Makers of Overkill are Trying Someth...
Craneballs, the studio responsible for the Overkill series, is taking a little break from all that violence (a little break) to bring us Cube Worm - a 3D take on one of the most classic PC/calculator games in existence. [Read more] | Read more »
The Sandbox Welcomes Cutethulu in its La...
Another month, another update to Pixowl's The Sandbox. This time players can say hello to "Cutethulu" - an adorable little rendition of perhaps the most well known (and infamous) of the Elder Gods. [Read more] | Read more »
ReBoard: Revolutionary Keyboard (Utilit...
ReBoard: Revolutionary Keyboard 1.0 Device: iOS Universal Category: Utilities Price: $1.99, Version: 1.0 (iTunes) Description: Do everything within the keyboard without switching apps! If you are in WhatsApp, how do you schedule a... | Read more »
Tiny Empire (Games)
Tiny Empire 1.1.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1.3 (iTunes) Description: Launch cannonballs and blow tiny orcs into thousands of pieces in this intuitive fantasy-themed puzzle shooter! Embark on an... | Read more »
Astropad Mini (Productivity)
Astropad Mini 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: *** 50% off introductory price! ​*** Get the high-end experience of a Wacom tablet at a fraction of the price with Astropad... | Read more »

Price Scanner via MacPrices.net

eFileCabinet Announces SMB Document Managemen...
Electronic document management (EDM) eFileCabinet, Inc., a hosted solutions provider for small to medium businesses, has announced that its SecureDrawer and eFileCabinet Online services will be... Read more
WaterField Designs Unveils American-Made, All...
San Francisco’s WaterField Designs today unveiled their all-leather Cozmo 2.0 — an elegant attach laptop bag with carefully-designed features to suit any business environment. The Cozmo 2.0 is... Read more
Apple’s 2015 Back to School promotion: Free B...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
128GB MacBook Airs on sale for $100 off MSRP,...
B&H Photo has 11″ & 13″ MacBook Airs with 128GB SSDs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $799.99, $100 off MSRP... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.... Read more
27-inch 3.2GHz iMac on sale for $1679, save $...
B&H Photo has the 27″ 3.2GHz iMac on sale for $1679.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Apple and Cisco Partner to Deliver Fast-Lane...
Apple and Cisco have announced a partnership to create a “fast lane” for iOS business users by optimizing Cisco networks for iOS devices and apps. The alliance integrates iPhone with Cisco enterprise... Read more
Apple offering refurbished 2015 13-inch Retin...
The Apple Store is offering Apple Certified Refurbished 2015 13″ Retina MacBook Pros for up to $270 (15%) off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple refurbished 2015 MacBook Airs available...
The Apple Store has Apple Certified Refurbished 2015 11″ and 13″ MacBook Airs (the latest models), available for up to $180 off the cost of new models. An Apple one-year warranty is included with... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $120 off MSRP including free shipping plus NY sales tax only: - 21″ 1.4GHz iMac: $999.99 $100 off - 21″ 2.7GHz iMac: $1199.99 $100 off - 21″ 2.9GHz iMac... Read more

Jobs Board

Simply Mac *Apple* Specialist- Repair Techn...
Simply Mac is the greatest premier retailer of Apple products expertise in North America. We're looking for dedicated individuals to provide personalized service and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the greatest premier retailer of Apple products expertise in North America. We're looking for dedicated individuals to provide personalized service and Read more
*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
Simply Mac- *Apple* Specialist- Store Manag...
Simply Mac is the largest premier retailer for Apple products and solutions. We're looking for dedicated individuals with a passion to simplify and enhance the Read more
*Apple* Evangelist - JAMF Software (United S...
The Apple Evangelist is responsible for building and cultivating strategic relationships with Apple 's small and mid-market business development field teams. This Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.