python-debugging-notes.md

Debugging Python Notes

Debugging with pdb

This is the Python debugger included in the standard library.

Usage:

import pdb

def buggy_method():
    pdb.set_trace()

When the code is executed, this will drop you into a pdb prompt at the trace point in your code.

At the prompt you can ask for help:

(Pdb) h

Documented commands (type help <topic>):
========================================
EOF    c          d        h         list      q        rv       undisplay
a      cl         debug    help      ll        quit     s        unt
alias  clear      disable  ignore    longlist  r        source   until
args   commands   display  interact  n         restart  step     up
b      condition  down     j         next      return   tbreak   w
break  cont       enable   jump      p         retval   u        whatis
bt     continue   exit     l         pp        run      unalias  where

Miscellaneous help topics:
==========================
exec  pdb

(Pdb) h a
a(rgs)
        Print the argument list of the current function.

Debugging with pudb

Pudb is more of a full fledged interactive, console based debugger.

Install:

$ python -m pip install pudb

Usage:

from pudb import set_trace

def buggy_method():
    set_trace()

Debugging with iPython

iPython is a feature rich repl, kind of like a more powerful version of Ruby's pry.

You can use it to debug by "embedding" it in your source. When you run your code, the embed() statement will launch an iPython terminal at that point in your code, at which point you can poke around.

Install:

$ python -m pip install ipython

Usage #1 (more repl than debug):

from IPython import embed

def buggy_method():
    set_trace()

However, this doesn't give you debug capabilities, like code listing and stepping into methods.

Usage #2 (more debug):

from IPython.core import debugger
debug = debugger.Pdb().set_trace

def buggy_method():
    debug()

Printing out an object

On a console you may want to print out an object.

Option #1: Convert it to a string, which calls the __str__ method.

str(my_object)

Option #2: Print out the object data in dictionary style.

vars(my_object)