Tip for debugging SymPy with PuDB

Usually, when I debug SymPy code with PuDB, I create a script that calls the code, then I put a

import pudb; pudb.set_trace()

in the SymPy library code where I want to start debugging. But this is annoying, first because I have to create the script, and second, because I have to modify the library code, and there’s always the risk of accidentally commiting that. Also, if I want to start debugging somewhere else, I have to edit the files and change it.

Well, I just figured out a better way. First, if you haven’t already, add an alias like this in your bash config file (~/.profile or ~/.bashrc):alias pudb='python -m pudb.run. As of this pull request, this is no longer necessary. A pudb script is installed automatically with PuDB.

This will let you run pudb script.py to debug script.py. Next, start PuDB. It doesn’t matter with what. You can just run touch test.py, and then pudb test.py. It occured to me that you can just set the breakpoint when starting isympy with PuDB.

Now, press m, and navigate to where in the library code you want to start debugging. It also helps to use / to search the current file and L to jump to a specific line. When you get to the line where you want to start debugging, press b to set a breakpoint. You can do this in multiple places if you want.

Now, you just have to start isympy from within PuDB. Just run pudb bin/isympy, and immediately press c to jump to the interactive prompt. Now, run whatever code you want to debug. When it gets to the breakpoint, PuDB will open, and you can start debugging. If you type c to continue, it will go back to isympy. But the next time you run something that hits the breakpoint, it will open PuDB again.

This trick works because breakpoints are saved to file (at ~/.config/pudb/saved-breakpoints). In fact, if you want, you can just modify that file in the first step. You can edit your saved breakpoints in the bottom right pane of PuDB.

When you are done and you type Ctrl-D PuDB will pop-up again, asking if you want to quit. That’s because it was running the whole time, underneath isympy. Just press q. Note that you should avoid pressing q while debugging, or else PuDB will quit, and you will be left with just normal isympy (it won’t break at your breakpoints any more). Actually, if you do this, but doing Ctrl-D still opens the PuDB prompt, you can just press “Restart”, and it should start working again. Note that “Restart” will not actually reset isympy: all your saved variables will still be the same, and any changes to the library code will not be reloaded. To do that, you have to completely exit and start over again.

Of course, there is nothing SymPy specific about this trick. As long as you have a script that acts as an entry point to an interactive console for your application, you can use it. If you just use IPython, you can use something like pudb /bin/ipython (replace /bin/ipython with the output of which ipython).