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

Monolingual 1.6.4 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. If you use your computer in only one (human) language, you... Read more
CleanApp 5.0 - Application deinstaller a...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Fantastical 2.0 - Create calendar events...
Fantastical is the Mac calendar you'll actually enjoy using. Creating an event with Fantastical is quick, easy, and fun: Open Fantastical with a single click or keystroke Type in your event details... Read more
Cocktail 8.2 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Direct Mail 4.0.4 - Create and send grea...
Direct Mail is an easy-to-use, fully-featured email marketing app purpose-built for OS X. It lets you create and send great looking email campaigns. Start your newsletter by selecting from a gallery... Read more
jAlbum Pro 12.6 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.6 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Lyn 1.5.9 - Lightweight image browser an...
Lyn is a lightweight and fast image browser and viewer designed for photographers, graphic artists and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it... Read more
Sublime Text 3080 - Sophisticated text e...
Sublime Text is a sophisticated text editor for code, markup, and prose. You'll love the slick user interface, extraordinary features, and amazing performance. Goto Anything. Use Goto Anything to... Read more
WALTR 1.0.11 - Drag-and-drop any media f...
WALTR is designed to make it easy to upload and convert any music or video file to an iPad or iPhone format for native playback. It supports a huge variety of media file types, including MP3, MP4,... Read more

MLB Manager 2015 (Games)
MLB Manager 2015 5.0.14 Device: iOS Universal Category: Games Price: $4.99, Version: 5.0.14 (iTunes) Description: Guide your favorite MLB franchise to glory! MLB Manager 2015, officially licensed by MLB.com and based on the award-... | Read more »
Breath of Light (Games)
Breath of Light 1.0.1421 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1421 (iTunes) Description: Hold a quiet moment. Breath of Light is a meditative and beautiful puzzle game with a hypnotic soundtrack by... | Read more »
WWE WrestleMania Tags into the App Store
Are You ready to rumble? The official WWE WrestleMania app, by World Wrestling Entertainment, is now available. Now you can get all your WrestleMania info in one place before anyone else. The app offers details on superstar signings, interactive... | Read more »
Bio Inc's New Expansion is Infectin...
Bio Inc., by DryGin Studios, is the real time strategy game where you infect a human body with the worst virus your evil brain can design. Recently, the game was updated to add a whole lot of new features. Now you can play the new “Lethal”... | Read more »
The Monocular Minion is Here! Despicable...
Despicable Me: Minion Rush, by Gameloft, is introducing a new runner to the mix in their latest update. Now you can play as Carl, the prankster minion. Carl has a few new abilities to play with, including running at a higher speed from the start.... | Read more »
Dungeon of Madness (Games)
Dungeon of Madness 1.0.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.0 (iTunes) Description: Dungeon of Madness is an action game where you rotate tiles to create our own route. Help the hero by connecting the... | Read more »
Filters for iPhone (Photography)
Filters for iPhone 1.0 Device: iOS iPhone Category: Photography Price: $.99, Version: 1.0 (iTunes) Description: | Read more »
Jump'N'Shoot Attack (Games)
Jump'N'Shoot Attack 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: A mobile game for gamers! Join Louise Lightfoot, the legendary "Master of Jumping and Shooting", on her mission to save... | Read more »
Space Bounties Inc. (Games)
Space Bounties Inc. 1.4 Device: iOS Universal Category: Games Price: $1.99, Version: 1.4 (iTunes) Description: SuperGameDroid: 4/5 "Satisfying futuristic RPG combat, high replay value, and a heavy dose of nostalgia make Space... | Read more »
Gamebook: Pocket RPG (Games)
Gamebook: Pocket RPG 1.0.11 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.11 (iTunes) Description: Walk into the Land of Lanthir Lamath ruled by wicked skeletons and fight for your life in a thrilling adventure.... | Read more »

Price Scanner via MacPrices.net

Logitech Says MX Master Is Its Most Advanced...
Logitech’s new MX Master Wireless Mouse incorporates the best of Logitech’s many computer mouse innovations into a striking hand-sculpted design. The company claims that the MX Master creates a new... Read more
Save up to $300 on a new Mac, $30 on an iPad,...
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
Apple refurbished 2014 MacBook Airs available...
The Apple Store lowered prices on Apple Certified Refurbished 2014 MacBook Airs recently, with models now available starting at $679. An Apple one-year warranty is included with each MacBook, and... Read more
Mac Notebook Evolution; A Desktop Replacement...
More often than not right from the beginning, Apple’s Macs have tended to skew toward small. The original Macs were called “compacts,”, and notwithstanding a few exceptions like the honking Big Mac... Read more
13-inch 1.4GHz/128GB MacBook Air (Apple refur...
The Apple Store has Apple Certified Refurbished 2014 13″ 1.4GHz/128GB MacBook Airs available for $759 including free shipping plus Apple’s standard one-year warranty. Their price is $240 off original... Read more
YEP! Alternative Browser for iOS Now Supports...
Pfaeffikon, Switzerland based Power App AG has announced the release of an update to their Yep! Web Browser (v1.3.0) for iOS8 iPhone and iPad. Yep! hit the App Store shortly after the release of iOS... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $250 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $... Read more
Clearance 13-inch Retina MacBook Pros availab...
B&H Photo has leftover 2014 13″ Retina MacBook Pros on sale for up to $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1098... Read more
Clearance 2014 MacBook Airs on sale for up to...
B&H Photo has MacBook Airs on sale for up to $180 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $789.99 110 off original MSRP - 11″ 256GB... Read more
Apple refurbished Time Capsules available for...
The Apple Store has certified refurbished Time Capsules available for $100 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $199, $100... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - D...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Systems Engineer - Pre Sales, Educat...
…is responsible for proactively providing technical expertise to drive sales of Apple solutions into assigned accounts. The SE architects, validates, and assists in Read more
Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.