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

Bookends 12.5.8 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Chromium 44.0.2403.125 - 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 44.0.2403.125: This release contains a number... Read more
iMazing 1.2.2 - Complete iOS device mana...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
Audio Hijack 3.2.0 - Record and enhance...
Audio Hijack (was Audio Hijack Pro) drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio... Read more
FontExplorer X Pro 5.0.1 - Font manageme...
FontExplorer X Pro is optimized for professional use; it's the solution that gives you the power you need to manage all your fonts. Now you can more easily manage, activate and organize your... Read more
Calcbot 1.0.2 - Intelligent calculator a...
Calcbot is an intelligent calculator and unit converter for the rest of us. Featuring an easy-to-read history tape, expression view, intuitive conversion, and much more! Features History Tape -... Read more
MTR 5.0.0.1 - The Mac's oldest and...
MTR (was MacTheRipper)--the Mac's oldest and smartest DVD-backup app--is now updated to version 5.001 MTR -- the complete toolbox, not a one-trick, point-and-click extractor. MTR is intended for... Read more
LibreOffice 4.4.5.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
Adobe Lightroom 6.1.1 - Import, develop,...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
File Juicer 4.41 - Extract images, video...
File Juicer is a drag-and-drop can opener and data archaeologist. Its specialty is to find and extract images, video, audio, or text from files which are hard to open in other ways. It finds and... Read more

Pac-Man Championship Edition DX has brou...
Bandai Namco has released Pac-Man Championship Edition DX on iOS and Android, which features the classic arcade gameplay that we've all grown to love. Pac-Man Championship Edition DX can be enjoyed in much shorter bursts than the arcade versions... | Read more »
Cosmonautica (Games)
Cosmonautica 1.1 Device: iOS Universal Category: Games Price: $6.99, Version: 1.1 (iTunes) Description: Cast off! Are you ready for some hilarious adventures in outer space? | Read more »
Rescue humanity from a Demon horde in An...
Angel Stone is Fincon's follow up to the massively successful Hello Hero and is out now on iOS and Android. You play as a member of The Resistance, a group of mighty human warriors who have risen up in defiance of the Demon horde threatening to... | Read more »
Gallery Doctor (Photography)
Gallery Doctor 1.0 Device: iOS iPhone Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Free up valuable iCloud and iPhone storage with Gallery Doctor, the only iPhone cleaner that automatically identifies the... | Read more »
You Against Me (Games)
You Against Me 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: A simple game… You. Me. Claim, steal, lock, score, win! | Read more »
Yep, it's True - Angry Birds 2 is O...
The not exactly rumors were true and the birds are back. Angry Birds 2 has come to the App Store and the world will... well I suppose it'll still be the same, but now we have more bird-flinging options! [Read more] | Read more »
You Could Design Your Own Card for Chain...
If you've ever wanted to create your own item, weapon, trap, or even monster for Chainsaw Warrior: Lords of the Night, this is your chance. Auroch Digital is currently holding a contest so that fans can fight to the death (not really) to see which... | Read more »
Bitcoin Billionaire is Going Back in Tim...
If you thought you managed to buy everything there is to buy in Bitcoin Billionaire and make all the money, well you though wrong. Those of you who made it far enough might remember investing in time travel - and it looks like that investment is... | Read more »
Domino Drop (Games)
Domino Drop 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Domino Drop is a delightful new puzzle game with dominos and gravity!Learn how to play it in a minute, master it day by day.Your... | Read more »
OPERATION DRACULA (Games)
OPERATION DRACULA 1.0.1 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0.1 (iTunes) Description: 25% off launch sale!!! 'Could prove to be one of the most accurate representations of the Japanese bullet hell shmup... | Read more »

Price Scanner via MacPrices.net

11-inch MacBook Airs on sale for $100 off MSR...
Best Buy has 11-inch MacBook Airs on sale for $100 off MSRP. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - 11″ 1.6GHz... Read more
iPad Air 2 on sale for up to $100 off MSRP
Best Buy has iPad Air 2s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices available for online... Read more
Sale! 13-inch MacBook Pros on sale for $100 o...
B&H Photo has 13″ MacBook Pros on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.5GHz/500GB MacBook Pro: $999.99 save $100 - 13″ 2.7GHz/128GB Retina... Read more
Sale! Save $100 on 13-inch MacBook Airs this...
B&H Photo has the 13″ 1.6GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model.... Read more
Worldwide Tablet Market Decline Continues, Ap...
The worldwide tablet market declined -7.0% year-over-year in the second quarter of 2015 (2Q15) with shipments totaling 44.7 million units according to preliminary data from the International Data... Read more
TP-LINK TL-PA8030P KIT Powerline Featuring Ho...
Consumer and business networking products provider TP-LINK is now shipping its TL-PA8030P KIT AV1200 3-Port Gigabit Passthrough Powerline Starter Kit that expands your home’s network over its... Read more
Apple refurbished iPad Air 2s available for u...
The Apple Store has Apple Certified Refurbished iPad Air 2s available for up to $140 off the price of new models. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB... Read more
Updated Apple iPad Price Trackers
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers. Read more
Apple refurbished 2014 13-inch 128GB MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 13″ 1.4GHz/128GB... Read more
Apple’s Education discount saves up to $300 o...
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

Jobs Board

Infrastructure Engineer - *Apple* /Mac - Hil...
Infrastructure Engineer - Apple /Mac Job Code: 1608 # of openings: 1 Description Our fortune 500 client is looking to hire an experienced Infrastructure Engineer to join Read more
Executive Administrative Assistant, *Apple*...
…supporting presentation development for senior leadership. * User experience with Apple hardware and software is preferred. Additional Requirements The following list Read more
*Apple* Bus Company is now hirin - Apple Bus...
Apple Bus Company is now hiring school bus drivers in the Pettis County area. Class B CDL preferred. Free training provided. No nights or weekends required. Flexible Read more
*Apple* Certified Mac Technician - Updated 6...
…and friendly, hands-on technical support to customers troubleshooting and repairing Apple /Mac products with courtesy, speed and skill. Use your problem-solving skills Read more
Infrastructure Engineer - *Apple* /Mac Envir...
…a part of a team Requires proven problem solving skills Preferred Additional Apple Certified System Administrator (ACSA) Apple Certified Technical Coordinator (ACTC) Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.