TweetFollow Us on Twitter

Mac in the Shell: Debugging Python with vim

Volume Number: 26
Issue Number: 01
Column Tag: Mac in the Shell

Mac in the Shell: Debugging Python with vim

IDE-like debugging without leaving vim

by Edward Marczak

Welcome

Mac in the Shell has been focusing on Python for the System Administrator over the last few months. Last month re-introduced the vi(m) text editor. This month, we're going to tie them together a bit. I've covered pdb - the python debugger - already. It's really handy to have a debugger at your disposal. However, pdb's output and display can get a little unwieldy. Never fret: Vim to the rescue! This article will show you how to get the latest MacVim and start using it to debug Python code.

Use the Source, Luke

First thing is first: this will all fail miserably with the version of vim that's built into the base OS. Best solution: use MacVim. MacVim comes built with Python, GUI support and more. There's a few ways to get MacVim, and I'll detail both.

First way is the easy way: download a binary release, typically of the latest snapshot. Do that directly from the MacVim project page, currently on Google Code:

http://code.google.com/p/macvim/

But wait, there's more! If you like to customize your experience, like I do, you'll want the source code. This is a multi-part step. First, you'll need to make sure that the (free) Apple developer tools are installed on your machine. (and honestly, what good is a machine without those?). You'll also need git installed to check out the MacVim source. If you're a fink or MacPorts person, you can use either of those systems to install git. If you're a pre-built person, or just need to be in this case, go pick up a pre-built git from:

http://code.google.com/p/git-osx-installer/

Once git is up and running, check out the MacVim source by changing to the destination directory and issuing a clone command:

git clone git://repo.or.cz/MacVim.git vim7

Once that completes, you'll need to configure and build MacVim. This is where you can customize the binary a bit. I want the whole experience, so, I change to the vim7 directory and run configure with my options:

./configure 
—enable-perlinterp —enable-pythoninterp 
—enable-rubyinterp 
—with-features=huge 
—enable-gui=macvim 
—with-tlib=ncurses 
—enable-multibyte —with-python-config-dir=
/System/Library/Frameworks/Python.framework/Versions/2.6//lib/python2.6/config/

No matter how you configure Vim, I'd highly recommend the -enable-gui option and, for this article, you must enable the python interpreter. Of course, you're reading an article about debugging python using Vim, so why wouldn't you enable this?

Once configure finishes its business (successfully), type make and...wait. Barring any errors, you'll find the MacVim application in the src/MacVim/build/Release directory. If you're following this method, when you want to 'upgrade' (read: compile a new version), change into the vim7 directory and ask git to pull down the latest changes:

$ git pull

followed by make. Go pick up your fresh binary in the Release directory as before.

You may also want to copy the mvim script from the src/MacVim subdirectory and place it somewhere in your $PATH. Running mvim from the command-line will try to do the right thing: if you're logged in via ssh, you'll use console-based vim. If you have a GUI, you'll run GUI-based MacVim. You can also create alias or functions that call mvim as a different name. This will cause Vim to run in different modes. I have the following in my ~/.bash_profile initialization script:

export VIM_APP_DIR=${HOME}/Applications/MacVim/
vi() { ${HOME}/Applications/MacVim/mvim ${@}; }
vim() { ${HOME}/Applications/MacVim/mvim ${@}; }
gvim () { ${HOME}/Applications/MacVim/mvim ${@}; }
vimdiff() { ${HOME}/Applications/MacVim/mvim -d ${@}; }
export -f vi
export -f vim
export -f gvim
export -f vimdiff

I use bash functions rather than aliases as there are several problems with using aliases in this case. Primarily, a bash alias will fork off a new shell and there's too much juggling around of variables. Bash aliases have been deprecated in favor of bash functions in any case.

In my example .bash_profile snippet, I explicitly define where MacVim lives as I keep it in my home directory. Then I define the four ways I may call vi(m). One of the most important to me is the definition for vimdiff. When integrating with version control programs, I want to make sure it doesn't fork off, and it should come back after an individual window is closed. Once these are defined, I separately do the same for gvim and git integration if I'm logged in to a GUI session with:

export GIT_EDITOR='gvim -f -c "au VimLeave * !open -a Terminal"'

The final group of exports causes functions to be exported to subshells.

One last note: no matter how you choose to call vim, ensure that your method gets found before the system vi(m). Keep it early in your path, or use my method above, as functions are always used before searching $PATH.

The Setup

The entire point of the earlier Vim exercise is to ensure that you are using a Python-enabled version of vim. The vim binary that ships with OS X is not compiled with the Python, or any scripting option. Once you've downloaded or compiled MacVim and have it in place, launch it and test for Python support. Enter ex mode by typing a colon (":") followed by py and a python command. Something like:

:py print 6*7

You'll see the result on the command line. Not exceptionally exiting, perhaps...not at the outset, anyway. However, having Python support built in means that Python can be used to script just about anything. That is exciting! This is also, not coincidentally, how we're going to be able to debug python code entirely within vim. If you receive an error from 'py' - specifically "E319: Sorry, the command is not available in this version" - ensure that you're running the updated version of vim that you downloaded (or built!) and not the system vi(m).

Configuring Vim for Debugging

There are plenty of hoops one can jump through to debug directly in vim. My favorite, though, is a freely downloadable plugin. Go grab vimpdb from its project page:

http://code.google.com/p/vimpdb/

From there, click on the download link. Or, you can just grab the source directly using subversion:

svn checkout http://vimpdb.googlecode.com/svn/trunk/ vimpdb

Pleasantly, subversion is built into OS X as of 10.5, so, you can just run that command with no fuss.

However you've obtained vimpdb, you need to drop VimPdb.py and VimPdb.vim into your vim plugin directory (~/.vim/plugin). Now you're ready...maybe.

By default, vimpdb uses function keys for instructing it on what action to take. If you're using a 'real' keyboard, you can likely just use the default key bindings. By 'real' keyboard, I'm essentially talking about any stand-alone keyboard. It's the keyboards that are built into the MacBooks and MacBook Pro machines that are the problem. If you're like me, you'll occasionally want to debug on the go. The trouble starts with the way Apple overloads the function keys. Despite a setting in System Preferences that allows you to "use all" function keys "as standard function keys," this doesn't do what you expect initially. You'll still need to undo those key settings in the "Keyboard Shortcuts" section of the Keyboard pref pane. It's about picking your battles, I suppose. However, those shortcuts are really nice to have. (I know, this is a fairly lame argument, but, hey - I'm the end-user here and I demand satisfaction). I've experimented a bit with creating a wrapper script for vim that swaps out the plist file that controls this (~/Library/Preferences/com.apple.symbolichotkeys.plist) with one that has no F-key definitions, but that causes other issues for me. Ultimately, I reached a compromise and only undo a few default keyboard shortcuts: gone are ^F2, ^F4 and ^F8. So, let's go set up our vim key mappings.


Figure 1 - Assigning - or unassigning - keyboard shortcuts.

Again, if you want to leave the default keys for vimpdb and disable the OS X integrated keys in the Keyboard pref pane, feel free. For this article, I'm going to make some very simple changes, but you can set up the keys as you like. Open up the vimpdb.vim file that you just placed in your ~/.vim/plugin directory (using Vim, of course!). Move to line 540 (in Vim, type "540 G"). I've altered these lines to map F3 and F4 to step into and step over, respectively. They now read:

   " Was F7 and F8
   map <buffer> <silent> <F3> :call PdbStepInto()<CR>
   map <buffer> <silent> <F4> :call PdbStepOver()<CR>

Also, I've needed to change out the places that F3 and F4 were already used. Scroll down to line 560 or so, and make these changes:

   " Was F4
   map <buffer> <silent> <F7> :call PdbEvalCurrentWord()<CR>
   map <buffer> <silent> <C-F7> :call PdbEvalCurrentWORD()<CR>
   " Was F3
   map <buffer> <silent> <F8> :call PdbEvalExpression()<CR>

Save the file, and you're ready.

Lock and Load

The default vimpdb key binding along with the changes we've made leave us with the following keys for controlling a debug session:

Start and stop debugging

- F5 - Start/continue debug session of current file.

- Ctrl-F5 - Start debugging and do not pause at first line

- Ctrl-Shift-F5 - Start debugging with a given list of parameters.

- Shift-F5 - Stop the current debug session.

- Ctrl-Alt-Shift-F5 - Restart the current debug session.

Breakpoints

- F2 - Toggle breakpoint.

- Ctrl-F2 - Toggle conditional breakpoint

- Shift-F2 - Toggle temporary breakpoint

- Ctrl-Shift-F2 - Clear all breakpoints in current file

- Ctrl-Alt-Shift-F2 - Clear all breakpoints in all files

- F11 - Print condition of conditional breakpoint under the cursor

Step/continue

- F3 - Step into

- F4 - Step over

- Ctrl-F4 - Continue running until reaching a return from function

Cursor movement

- F6 - Move cursor to currently debugged line.

- Ctrl-F6 - Change current debugged line to where the cursor is currently placed.

Stack

- F9 - Move up in stack frame.

- F10 - Move down in stack frame.

- F12 - Print stack trace

Evaluate/Execute

- F7 - Evaluate a given expression (in the current debug context)

- Ctrl-F7 - Exec a given statement (in the current debug context)

- F8 - Evaluate the current word under the cursor (in the current debug context)

- Ctrl-F8 - Evaluate the current WORD under the cursor (in the current debug context)

Again, depending on the keyboard you use and the setting you have in Keyboard Shortcuts, some of these key mappings may (will?) have conflicts.

So, what does all of this get you? Figure 2 should give you an idea.


Figure 2 - VimPDB in action.

Unlike the pure command-line environment of pdb, vimpdb lets you keep your code in view and step along with it as it runs. Visually. In figure 2, line 3 - highlighted in green - is the current line. Line 9 - highlighted in red - has a breakpoint set on it. You can use any Python code that you like. I'm going to use the code that I offered up in my article on PyObjC from August 2009 that reads the Apple Address Book.

Debugging

Once you're set up, the debugging is fairly easy. Load the python code that you'd like to debug into the current buffer and press F5. This begins the debugger, and vim should print a status message that says "Starting debugger." You should also now see your first executable line highlighted in green. Now, the "first executable" line is the first thing that the parser needs to look at, not necessarily what you'd expect as "executable."

To step through the program, you can press F4 to step 1 instruction. This will step over any intervening code between the current line and what is visible as next. The current line will execute, and the green line will advance. If the current line is a function call that you want to inspect, press F3 to step into the function. You'll need to do this at least once if you use the Python convention of creating a main() function and calling that conditionally:

if __name__ == '__main__':
  main()

Once you're inside a function, often, you'll get to the part you're interested in, and then not want to have to step through the remainder of the function. You can accomplish that by pressing control-F4.

The great thing about using a debugger is not only can you step through and watch the flow of your code, but you can inspect variables as they change, too. Vimpdb doesn't let us down here. Position the cursor over the variable name you want to inspect and press F7. The value of the variable is displayed in the output area at the bottom of the vim screen followed by a note to press any key to return to debugging.

For example, if I were to press F7 over the "abgroups" variable showing in Figure 2, I receive this output:

 (
    "ABGroup (0x10835cb40) {\n\tCreation     : 2009-07-14 15:13:36 
    -0400\n\tGroupName    : WS\n\tModification : 2009-10-27 19:24:14 
    -0400\n\tUnique ID    : 8BBAEDE9-0D31-4B48-A967-826523BF9DA7:ABGroup\n}",
    "ABGroup (0x1006f3ab0) {\n\tCreation     : 2009-01-14 15:13:36 
    -0400\n\tGroupName    : MacTech Editorial\n\tMembers    
    : 28 members\n\tModification : 2009-11-23 08:04:24 
    -0500\n\tUnique ID    : 5712EB9A-9743-4744-BA05-354726896614:ABGroup\n}"
)

Note that variables you inspect must be defined. If the current line contains a variable that hasn't been defined yet, you can't inspect that variable until the line has been executed. If you try, you'll receive a NameError:

NameError: Name 'blah' is not defined.

Just ensure that the variable you're trying to inspect is defined. If it's first defined on the current line, execute the current line first.

Let's look at a typical short debug session. First, you'd open vim and load your code into the buffer:

vim ~/dev/py/address_book.py

(or, open vim and type :e ~/dev/py/address_book.py).

Start the debug session by pressing F5 to initialize the debugger and start the session. You should note the "starting debugger" message. No other vimpdb keys will do anything until this step is taken.

You can navigate down to the call to main() and set a breakpoint - using F2 while the cursor is on the line with said call. Once set, press F5 to run the program as usual until it hits your break point. If you stopped on main(), press F4 to step into the main function. Execute a line that assigns a value to a variable, cursor up to the variable name and press F7 to inspect the value that was assigned; it'll appear at the bottom of the vim window. Press F3 to execute the current line.

Rinse, repeat.

One extra nice feature of VimPDB: the ability to save and load breakpoints. In a larger program, you may set several breakpoints in different locations that you want to recall after quitting vim and then trying to debug again. While in debug mode with all of the breakpoints set, type \s (that's backslash, and then s). You'll be prompted for a filename. Type one and press return. When you later come back to vim and load your project, start the debugger and type \l (backslash, then lowercase L, for "load").

In Closing

VimPDB provides the best of all worlds for debugging Python code. It doesn't make you change code to drop in a bunch of print or logging statements. It gives you a larger view of your code as it's running. Finally, it can easily be run remotely over an ssh session (win!).

Also, VimPDB is pretty easy in general, and a great addition to your toolkit. This article focused mainly on the preparation: install and introduction. Once you start using it, it becomes second nature. Unfortunately, unless you wildly remap the default keys, you'll need to choose between OS X functions on the F-keys and bindings in Vim (and yes, OS X "wins"). This is of particular annoyance on a MacBook/Pro, but not insurmountable.

Media of the month: No better time to recommend "Learning the vi and Vim Editors" by Arnold Robbins, Elbert Hannah, and Linda Lamb. This is a pretty comprehensive look at learning and customizing vi(m). Available in dead-tree and electronic versions.

Hope to see everyone in San Francisco for Macworld!


Ed Marczak is the Executive Editor of MacTech Magazine. He has written the Mac in the Shell column since 2004.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Artlantis Studio 5.1.2.7 - 3D rendering...
Artlantis Studio is a unique and ideal tool for performing very high resolution rendering easily and in real time. The new FastRadiosity engine now lets you compute images in radiosity-even in... Read more
MacUpdate Desktop 6.0.5 - Search and ins...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
BitTorrent Sync 2.0.82 - Sync files secu...
BitTorrent Sync allows you to sync unlimited files between your own devices, or share a folder with friends and family to automatically sync anything. File transfers are encrypted. Your information... Read more
Google Drive 1.20 - File backup and shar...
Google Drive is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a wedding with your fiancé, or... Read more
Simon 4.0.3 - Monitor changes and crashe...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
Vitamin-R 2.23 - Personal productivity t...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
iDefrag 5.0.0 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more
PCalc 4.2 - Full-featured scientific cal...
PCalc is a full-featured, scriptable scientific calculator with support for hexadecimal, octal, and binary calculations, as well as an RPN mode, programmable functions, and an extensive set of unit... Read more
FileZilla 3.10.2 - Fast and reliable FTP...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.10.2: Note: Now requires a 64-bit Intel processor.... Read more
The Hit List 1.1.11 - Advanced reminder...
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

Protect Yourself from an Onslaught of Ca...
Surprise Attack Games has announced a Cat-astrophic new physics puzzler called Fort Meow! In the game, a young girl named Nia finds her grandfather’s journal which triggers an all mighty feline attack! Why do the cats want the journal? Who knows,... | Read more »
GDC 2015 – Jelly Reef will be Game Oven’...
GDC 2015 – Jelly Reef will be Game Oven’s Last Hurrah, and it Seems like a Good Note to Go Out on Posted by Rob Rich on March 4th, 2015 [ permalink ] It’s sad knowing that Game Oven ( | Read more »
daWindci Deluxe Review
daWindci Deluxe Review By Campbell Bird on March 4th, 2015 Our Rating: :: BLUSTERY PUZZLESUniversal App - Designed for iPhone and iPad This updated puzzle game offers some creative gameplay and new mechanics, but still suffers from... | Read more »
Dungeon Hunter 5 Coming on March 12
Gameloft has excitedly announced that Dungeon Hunter 5 is on its way! Once again, you will adventure across the land of Valenthia exploring dungeons and fighting monsters. The game will have a new asynchronous multiplayer mode called Strongholds... | Read more »
GDC 2015 – The Sandbox 2 is Coming, and...
GDC 2015 – The Sandbox 2 is Coming, and Now it has Textures! | Read more »
Warner Bros. Interactive Announces Mort...
Mortal Kombat X, by Warner Bros. and NetherRealm Studios, will be a a free-to-play fighting/card-battle Mortal Kombat game. The game promises card collecting, multiplayer team combat, classic characters such as Scorpion, Sub-Zero and Raiden, and the... | Read more »
GDC 2015 – Piloteer is Whitaker Trebella...
GDC 2015 – Piloteer is Whitaker Trebella’s Latest Project, and it’s Definitely Something DIfferent Posted by Rob Rich on March 3rd, 2015 [ permalink ] You know | Read more »
PangoLand Review
PangoLand Review By Amy Solomon on March 3rd, 2015 Our Rating: :: COME VISIT PANGO AND FRIENDSUniversal App - Designed for iPhone and iPad PangoLand is an open-ended world full of familiar characters, bright colors and interactive... | Read more »
Knights of Pen & Paper is Leveling U...
With the roll of a die and a critical success, Paradox Interactive has announced Knights of Pen & Paper 2! You’ll be taking your place at the table once again in this sequel to Knights of Pen & Paper +1 Edition. The game will introduce the... | Read more »
GDC 2015 – Project Highrise is an Intere...
GDC 2015 – Project Highrise is an Interesting Idea from SomaSim Posted by Rob Rich on March 3rd, 2015 [ permalink ] You might know SomaSim best from their gold rush sim, 1849< | Read more »

Price Scanner via MacPrices.net

Mac Pros on sale for up to $279 off MSRP
Amazon has Mac Pros in stock and on sale for up to $279 off MSRP. Shipping is free: - 4-Core Mac Pro: $2725.87, $273 off MSRP (9%) - 6-Core Mac Pro: $3719.99, $279 off MSRP (7%) Read more
Sale! 13-inch Retina MacBook Pros for up to $...
B&H Photo has 13″ Retina MacBook Pros on sale for up to $205 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1219.99 save $80 - 13″ 2.... Read more
Another Tranche Of IBM MobileFirst For iOS Ap...
IBM has announced the next expansion phase for  its IBM MobileFirst for iOS portfolio, with a troika of new apps to address key priorities for the Banking and Financial Services, Airline and Retail... Read more
Sale! 15-inch Retina MacBook Pros 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
WaterField Designs Introduces the Minimalist...
With Apple Pay gaining popularity, Android Pay coming in May 2015, and loyalty cards and receipts that can be accessed from smartphones, San Francisco’s WaterField Designs observes that it may be... Read more
Sale! 15-inch 2.2GHz Retina MacBook Pro for $...
 Best Buy has the 15″ 2.2GHz Retina MacBook Pro on sale for $1774.99 $1799.99, or $225 off MSRP. Choose free home shipping or free local store pickup (if available). Price valid for online orders... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $170 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz... Read more
13-inch 2.5GHz MacBook Pro on sale for $100 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
27-inch 3.5GHz 5K iMac in stock today and on...
 B&H Photo has the 27″ 3.5GHz 5K iMac in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available for... Read more
Apple Launches Free Web-Based Pages and Other...
Apple’s new Web-only access to iWork productivity apps is a free level of iCloud service available to anyone, including people who don’t own or use Apple devices. The service includes access to Apple... 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
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
*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* Pay Automation Engineer - iOS System...
**Job Summary** At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring passion and dedication to your job Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.