Debugging Python(Twisted) with Emacs

  • Open up your project files. sometimes emacs can’t find them if you don’t have them open before-hand.

  • Make sure you have a program called pdb somewhere in your PATH, with the following contents:

    #!/bin/sh
exec python -m pdb $1 $2 $3 $4 $5 $6 $7 $8 $9

  • Run M-x pdb in emacs. If you usually run your program as python foo.py , your command line should be pdb foo.py , for twistd and trial just add -b to the command line, e.g.: twistd -b -y  my.tac

  • While pdb waits for your input, go to a place in your code and hit C-x SPC to insert a break-point. pdb should say something happy. Do this in as many points as you wish.

  • Go to your pdb buffer and hit c ; this runs as normal until a break-point is found.

  • Once you get to a breakpoint, use s to step, n to run the current line without stepping through the functions it calls, w to print out the current stack, u and d to go up and down a level in the stack, p foo to print result of expression foo .

  • Recommendations for effective debugging:

    • use p self a lot; just knowing the class where the current code is isn’t enough most of the time.

    • use w to get your bearings, it’ll re-display the current-line/arrow

    • after you use w , use u and d and lots more p self on the different stack-levels.

    • If you’ve got a big code-path that you need to grok, keep another buffer open and list the code-path there (e.g., I had a nasty-evil Deferred recursion, and this helped me tons)

Footnotes