# 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