Skip to main content

Python: Coding in the Debugger for Beginners

Python has a wonderful interactive interpreter (i.e. shell). IPython is a third-party Python shell that's even nicer, but that's a topic for another post. It's fairly common to code in the shell until you have the code working correctly, and then copy-and-paste it into your program. Developing super-interactively is a great way to keep bugs at bay.

However, sometimes you need more setup before you can start coding. For instance, when writing a Web app in, say Pylons, you might need an actual request and a database connection before you can start coding what you want. You might even need a form POST before you can start. Ideally, you'd be able to start the shell from in the middle of your application at just the right spot. I'm pretty sure that someone out there knows how to get IPython to do the right thing, but I find using pdb, the Python debugger, really helpful for this purpose.

First of all, add the following wherever you want to break into the debugger, "import pdb; pdb.set_trace()". If you need some more help about the debugger itself, type "?". Now, you'll want to see what variables you have to play with, so use "dir()". Let's say the "request.environ" object has what you want, but you're not sure under what key. Well, start by printing it, "p request.environ". If you need a nicer looking version, try "from pprint import pprint; pprint(request.environ)". If you find the right key, say 'spam', then print that, "pprint(request.environ['spam'])". Maybe spam is an object, and you need to know what methods it has, "pprint(dir(request.environ['spam']))".

In the Python shell, you can use "help(x)" to find out more about x if x is a module, class, or function. If x is an object, try "help(x.__class__)". In the debugger, help() tends to not work for a reason that someone smarter than me probably understands. However, you can still do things like "p x.__class__.__doc__".

The debugger isn't very good for writing new functions or multi-line statements, but it's great for evaluating simple expressions. Once you have the code you want, you can copy-and-paste it back into your program and repeat the process. Writing code in this way is really helpful so that you can deal with the details one at a time without trying to debug a larger program that might have more than one bug. That's not to say I don't make heavy use of print-statement debugging, but using pdb is great if you want to "look around."


Anonymous said…
The pretty printer can be invoked in the debugger directly, without the import, by the 'pp' command
Paddy3118 said…
Anonymous must have pp autoloaded as part of his setup.
The default does not have the prettyprinter loaded.
- Paddy.
Unknown said…
pp works for me too, I don't remember having to configure anything for that. Just looked at my .pdbrc -- lots of other stuff but not pp.

pp is in the standard list of debugger commands.
jjinux said…
> The pretty printer can be invoked in the debugger directly, without the import, by the 'pp' command

Oh, helpful. Thanks!
Jordan G said…
Since you mentioned IPython, I thought I'd drop this in here:

Its really easy to embed IPython into your programs and use them in a similar (but MUCH more powerful way) to the debugger.
Just put:
from IPython.Shell import IPShellEmbed
at the top of your program, and when you would break into the debugger, run:

This allows you to poke at the state of your program with all the power of IPython!
jjinux said…
> This allows you to poke at the state of your program with all the power of IPython!

Awesome! That's the magical incantation I was looking for!

Popular posts from this blog

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python. Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening.

The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for Bukkit s…

Apple: iPad and Emacs

Someone asked my boss's buddy Art Medlar if he was going to buy an iPad. He said, "I figure as soon as it runs Emacs, that will be the sign to buy." I think he was just trying to be funny, but his statement is actually fairly profound.

It's well known that submitting iPhone and iPad applications for sale on Apple's store is a huge pain--even if they're free and open source. Apple is acting as a gatekeeper for what is and isn't allowed on your device. I heard that Apple would never allow a scripting language to be installed on your iPad because it would allow end users to run code that they hadn't verified. (I don't have a reference for this, but if you do, please post it below.) Emacs is mostly written in Emacs Lisp. Per Apple's policy, I don't think it'll ever be possible to run Emacs on the iPad.

Emacs was written by Richard Stallman, and it practically defines the Free Software movement (in a manner of speaking at least). Stal…

JavaScript: Porting from react-css-modules to babel-plugin-react-css-modules (with Less)

I recently found a bug in react-css-modules that prevented me from upgrading react-mobx which prevented us from upgrading to React 16. Then, I found out that react-css-modules is "no longer actively maintained". Hence, whether I wanted to or not, I was kind of forced into moving from react-css-modules to babel-plugin-react-css-modules. Doing the port is mostly straightforward. Once I switched libraries, the rest of the port was basically:
Get ESLint to pass now that react-css-modules is no longer available.Get babel-plugin-react-css-modules working with Less.Get my Karma tests to at least build.Get the Karma tests to pass.Test things thoroughly.Fight off merge conflicts from the rest of engineering every 10 minutes ;) There were a few things that resulted in difficult code changes. That's what the rest of this blog post is about. I don't think you can fix all of these things ahead of time. Just read through them and keep them in mind as you follow the approach above.…