Python debugging tips

Question

What are your best tips for debugging Python  Please don t just list a particular debugger without saying what it can actually do  Related  What are good ways to make my Python code run first time  - This discusses minimizing errors

User · Answer

http   pypi python org pypi pudb  a full-screen  console-based Python debugger      Its goal is to provide all the niceties of modern GUI-based debuggers in a more lightweight and keyboard-friendly package  PuDB allows you to debug code right where you write and test it     in a terminal  If you ve worked with the excellent  but nowadays ancient  DOS-based Turbo Pascal or C tools  PuDB s UI might look familiar      Nice for debugging standalone scripts  just run  python -m pudb run my-script py

User · Answer

PDB  You can use the pdb module  insert pdb set trace   anywhere and it will function as a breakpoint    gt  gt  gt  import pdb  gt  gt  gt  a  a string   gt  gt  gt  pdb set trace   --Return--  gt   lt stdin gt  1  lt module gt   - gt None  Pdb  p a  a string   Pdb    To continue execution use c  or cont or continue    It is possible to execute arbitrary Python expressions using pdb  For example  if you find a mistake  you can correct the code  then type a type expression to have the same effect in the running code  ipdb is a version of pdb for IPython  It allows the use of pdb with all the IPython features including tab completion   It is also possible to set pdb to automatically run on an uncaught exception   Pydb was written to be an enhanced version of Pdb  Benefits

User · Answer

In Vim  I have these three bindings   map  lt F9 gt  Oimport rpdb2  rpdb2 start embedded debugger  asdf    BREAK lt esc gt  map  lt F8 gt  Ofrom nose tools import set trace  set trace    BREAK lt esc gt  map  lt F7 gt  Oimport traceback  sys  traceback print exception  sys exc info     TRACEBACK lt esc gt    rpdb2 is a Remote Python Debugger  which can be used with WinPDB  a solid graphical debugger  Because I know you ll ask  it can do everything I expect a graphical debugger to do     I use pdb from nose tools so that I can debug unit tests as well as normal code   Finally  the F7 mapping will print a traceback  similar to the kind you get when an exception bubbles to the top of the stack   I ve found it really useful more than a few times

User · Answer

It is possible to print what Python lines are executed  thanks Geo    This has any number of applications  for example  you could modify it to check when particular functions are called or add something like    make it only track particular lines   code interact takes you into a interactive console  import code  code interact local locals      If you want to be able to easily access your console history look at   Can I have a history mechanism like in the shell    will have to look down for it    Auto-complete can be enabled for the interpreter

User · Answer

Defining useful repr   methods for your classes  so you can see what an object is  and using repr   or   r          or      0 r     format      in your debug messages logs is IMHO a key to efficient debugging    Also  the debuggers mentioned in other answers will make use of the repr   methods

User · Answer

print statements   Some people recommend a debug print function instead of print for easy disabling The pprint module is invaluable for complex structures

User · Answer

If you don t like spending time in debuggers  and don t appreciate poor usability of pdb command line interface   you can dump execution trace and analyze it later  For example   python -m trace -t setup py install  gt  execution log   This will dump all source line of setup py install execution to execution log   To make it easier to customize trace output and write your own tracers  I put together some pieces of code into xtrace module  public domain

User · Answer

Getting a stack trace from a running Python application  There are several tricks here  These include   Breaking into an interpreter printing a stack trace by sending a signal Getting a stack trace out of an unprepared Python process Running the interpreter with flags to make it useful for debugging

User · Answer

When possible  I debug using M-x pdb in emacs for source level debugging

User · Answer

Winpdb is very nice  and contrary to its name it s completely cross-platform   It s got a very nice prompt-based and GUI debugger  and supports remote debugging

User · Answer

the obvious way to debug a script  python -m pdb script py    useful when that script raises an exception useful when using virtualenv and pdb command is not running with the venvs python version    if you don t know exactly where that script is  python -m pdb   which  lt python-script-name gt

User · Answer

If you are familiar with Visual Studio  Python Tools for Visual Studio is what you look for

User · Answer

if you want a nice graphical way to print your call stack in a readable fashion  check out this utility  https   github com joerick pyinstrument  Run from command line   python -m pyinstrument myscript py  args       Run as a module   from pyinstrument import Profiler  profiler   Profiler   profiler start      code you want to profile  profiler stop   print profiler output text unicode True  color True     Run with django   Just add pyinstrument middleware ProfilerMiddleware to MIDDLEWARE CLASSES  then add  profile to the end of the request URL to activate the profiler

User · Answer

PyDev  PyDev has a pretty good interactive debugger  It has watch expressions  hover-to-evaluate  thread and stack listings and  almost  all the usual amenities you expect from a modern visual debugger  You can even attach to a running process and do remote debugging   Like other visual debuggers  though  I find it useful mostly for simple problems  or for very complicated problems after I ve tried everything else  I still do most of the heavy lifting with logging

User · Answer

ipdb is like pdb  with the awesomeness of ipython

User · Answer

There is a full online course called  Software Debugging  by Andreas Zeller on Udacity  packed with tips about debugging      Course Summary      In this class you will learn how to debug programs systematically  how   to automate the debugging process and build several automated   debugging tools in Python       Why Take This Course       At the end of this course you will have a solid understanding about   systematic debugging  will know how to automate debugging and will   have built several functional debugging tools in Python       Prerequisites and Requirements      Basic knowledge of programming and Python at the level of Udacity   CS101 or better is required  Basic understanding of Object-oriented   programming is helpful    Highly recommended

User · Answer

If you are using pdb  you can define aliases for shortcuts   I use these     Ned s  pdbrc    Print a dictionary  sorted   1 is the dict   2 is the prefix for the names  alias p  for k in sorted  1 keys     print   s -15s   -80 80s       2  k repr  1 k       Print the instance variables of a thing  alias pi p   1   dict    1     Print the instance variables of self  alias ps pi self    Print the locals  alias pl p  locals   local     Next and list  and step and list  alias nl n  l alias sl s  l    Short cuts for walking up and down the stack alias uu u  u alias uuu u  u  u alias uuuu u  u  u  u alias uuuuu u  u  u  u  u alias dd d  d alias ddd d  d  d alias dddd d  d  d  d alias ddddd d  d  d  d  d

User · Answer

Logging  Python already has an excellent  built-in logging module  You may want to use the logging template here   The logging module lets you specify a level of importance  during debugging you can log everything  while during normal operation you might only log critical things   You can switch things off and on   Most people just use basic print statements to debug  and then remove the print statements   It s better to leave them in  but disable them  then  when you have another bug  you can just re-enable everything and look your logs over   This can be the best possible way to debug programs that need to do things quickly  such as networking programs that need to respond before the other end of the network connection times out and goes away   You might not have much time to single-step a debugger  but you can just let your code run  and log everything  then pore over the logs and figure out what s really happening   EDIT  The original URL for the templates was  http   aymanh com python-debugging-techniques  This page is missing so I replaced it with a reference to the snapshot saved at archive org  http   web archive org web 20120819135307 http   aymanh com python-debugging-techniques  In case it disappears again  here are the templates I mentioned   This is code taken from the blog  I didn t write it   import logging import optparse  LOGGING LEVELS     critical   logging CRITICAL                     error   logging ERROR                     warning   logging WARNING                     info   logging INFO                     debug   logging DEBUG   def main      parser   optparse OptionParser     parser add option  -l    --logging-level   help  Logging level     parser add option  -f    --logging-file   help  Logging file name      options  args    parser parse args     logging level   LOGGING LEVELS get options logging level  logging NOTSET    logging basicConfig level logging level  filename options logging file                        format    asctime s   levelname s    message s                         datefmt   Y- m- d  H  M  S        Your program goes here      You can access command-line arguments using the args variable   if   name         main       main     And here is his explanation of how to use the above   Again  I don t get the credit for this     By default  the logging module prints critical  error and warning messages  To change this so that all levels are printed  use       your-program py --logging debug   To send log messages to a file called debug log  use       your-program py --logging-level debug --logging-file debug log

[python] Python debugging tips

Examples related to python

Examples related to debugging