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.

 
AAPL
$105.22
Apple Inc.
+0.39
MSFT
$46.13
Microsoft Corpora
+1.11
GOOG
$539.78
Google Inc.
-4.20

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Server 4.0 - For OS X 10.10 Yosemit...
Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, develop software, host your own website, publish wikis, configure Mac, iPhone,... Read more
TotalFinder 1.6.12 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
BusyCal 2.6.3 - Powerful calendar app wi...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. BusyCal's unique... Read more
calibre 2.7 - Complete e-library managem...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Skitch 2.7.3 - Take screenshots, annotat...
With Skitch, taking, annotating, and sharing screenshots or images is as fun as it is simple.Communicate and collaborate with images using Skitch and its intuitive, engaging drawing and annotating... Read more
Delicious Library 3.3.2 - Import, browse...
Delicious Library allows you to import, browse, and share all your books, movies, music, and video games with Delicious Library. Run your very own library from your home or office using our... Read more
Art Text 2.4.8 - Create high quality hea...
Art Text is an OS X application for creating high quality textual graphics, headings, logos, icons, Web site elements, and buttons. Thanks to multi-layer support, creating complex graphics is no... Read more
Live Interior 3D Pro 2.9.6 - Powerful an...
Live Interior 3D Pro is a powerful yet very intuitive interior designing application. View Video Tutorials It has every feature of Live Interior 3D Standard, plus some exclusive ones: Create multi... Read more
The Hit List 1.1.7 - Advanced reminder a...
The Hit List manages the daily chaos of your modern life. It's easy to learn - it's as easy as making lists. And it's powerful enough to let you plan, then forget, then act when the time is right.... Read more
jAlbum Pro 12.2.4 - Organize your digita...
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

Latest Forum Discussions

See All

Rami Ismail Opens Up distribute​() for D...
Rami Ismail Opens Up distribute​() for Developers Posted by Jessica Fisher on October 24th, 2014 [ permalink ] Rami Ismail, Chief Executive of Business and Development at indie game studio | Read more »
Great Hitman GO Goes on Sale and Gets Ne...
Great Hitman GO Goes on Sale and Gets New Update – Say That Three Times Fast Posted by Jessica Fisher on October 24th, 2014 [ permalink ] | Read more »
Rival Stars Basketball Review
Rival Stars Basketball Review By Jennifer Allen on October 24th, 2014 Our Rating: :: RESTRICTIVE BUT FUNUniversal App - Designed for iPhone and iPad Rival Stars Basketball is a fun mixture of basketball and card collecting but its... | Read more »
Rubicon Development Makes Over a Dozen o...
Rubicon Development Makes Over a Dozen of Their Games Free For This Weekend Only Posted by Jessica Fisher on October 24th, 2014 [ permalink ] | Read more »
I Am Dolphin Review
I Am Dolphin Review By Jennifer Allen on October 24th, 2014 Our Rating: :: NEARLY FIN-TASTICUniversal App - Designed for iPhone and iPad Swim around and eat nearly everything that moves in I Am Dolphin, a fun Ecco-ish kind of game... | Read more »
nPlayer looks to be the ultimate choice...
Developed by Newin Inc, nPlayer may seem like your standard video player – but is aiming to be the best in its field by providing high quality video play performance and support for a huge number of video formats and codecs. User reviews include... | Read more »
Fighting Fantasy: Caverns of the Snow Wi...
Fighting Fantasy: Caverns of the Snow Witch Review By Jennifer Allen on October 24th, 2014 Our Rating: :: CLASSY STORYTELLINGUniversal App - Designed for iPhone and iPad Fighting Fantasy: Caverns of the Snow Witch is a sterling... | Read more »
A Few Days Left (Games)
A Few Days Left 1.01 Device: iOS Universal Category: Games Price: $3.99, Version: 1.01 (iTunes) Description: Screenshots are in compliance to App Store's 4+ age rating! Please see App Preview for real game play! **Important: Make... | Read more »
Toca Boo (Education)
Toca Boo 1.0.2 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0.2 (iTunes) Description: BOO! Did I scare you!? My name is Bonnie and my family loves to spook! Do you want to scare them back? Follow me and I'll... | Read more »
Intuon (Games)
Intuon 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Join the battle with your intuition in a new hardcore game Intuon! How well do you trust your intuition? Can you find a needle in a... | Read more »

Price Scanner via MacPrices.net

Weekend sale: 13-inch 128GB MacBook Air for $...
Best Buy has the 2014 13-inch 1.4GHz 128GB MacBook Air on sale for $849.99, or $150 off MSRP, on their online store. Choose free home shipping or free local store pickup (if available). Price valid... Read more
Nimbus Note Cross=Platform Notes Utility
Nimbus Note will make sure you never forget or lose your valuable data again. Create and edit notes, save web pages, screenshots and any other type of data – and share it all with your friends and... Read more
NewerTech’s Snuglet Makes MagSafe 2 Power Con...
NewerTech has introduced the Snuglet, a precision-manufactured ring designed to sit inside your MagSafe 2 connector port, providing a more snug fit to prevent your power cable from unintentional... Read more
Apple Planning To Sacrifice Gross Margins To...
Digitimes Research’s Jim Hsiao says its analysts believe Apple is planning to sacrifice its gross margins to save its tablet business, which has recently fallen into decline. They project that Apple’... Read more
Who’s On Now? – First Instant-Connect Search...
It’s nighttime and your car has broken down on the side of the highway. You need a tow truck right away, so you open an app on your iPhone, search for the closest tow truck and send an instant... Read more
13-inch 2.5GHz MacBook Pro on sale for $949,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $949.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $150 off MSRP. Price is... Read more
Save up to $125 on Retina MacBook Pros
B&H Photo has the new 2014 13″ and 15″ Retina MacBook Pros on sale for up to $125 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels... Read more
Apple refurbished Time Capsules available sta...
The Apple Store has certified refurbished Time Capsules available for up to $60 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $255... Read more
Textilus New Word, Notes and PDF Processor fo...
Textilus is new word-crunching, notes, and PDF processor designed exclusively for the iPad. I haven’t had time to thoroughly check it out yet, but it looks great and early reviews are positive.... Read more
WD My Passport Pro Bus-Powered Thunderbolt RA...
WD’s My Passport Pro RAID solution is powered by an integrated Thunderbolt cable for true portability and speeds as high as 233 MB/s. HighlightsOverviewSpecifications Transfer, Back Up And Edit In... Read more

Jobs Board

*Apple* Solutions Consultant - Apple Inc. (U...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.