How to debug in Django the good way

Question

So  I started learning to code in Python and later Django  The first times it was hard looking at tracebacks and actually figure out what I did wrong and where the syntax error was  Some time has passed now and some way along the way  I guess I got a routine in debugging my Django code  As this was done early in my coding experience  I sat down and wondered if how I was doing this was ineffective and could be done faster  I usually manage to find and correct the bugs in my code  but I wonder if I should be doing it faster   I usually just use the debug info Django gives when enabled  When things do end up as I thought it would  I break the code flow a lot with a syntax error  and look at the variables at that point in the flow to figure out  where the code does something other than what I wanted   But can this be improved  Are there some good tools or better ways to debug your Django code

User · Answer

i highly suggest to use PDB   import pdb pdb set trace     You can inspect all the variables values  step in to the function and much more  https   docs python org 2 library pdb html  for checking out the all kind of request response and hits to database i am using django-debug-toolbar https   github com django-debug-toolbar django-debug-toolbar

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Sometimes when I wan to explore around in a particular method and summoning pdb is just too cumbersome  I would add   import IPython  IPython embed     IPython embed   starts an IPython shell which have access to the local variables from the point where you call it

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There are a few tools that cooperate well and can make your debugging task easier   Most important is the Django debug toolbar   Then you need good logging using the Python logging facility   You can send logging output to a log file  but an easier option is sending log output to firepython   To use this you need to use the Firefox browser with the firebug extension  Firepython includes a firebug plugin that will display any server-side logging in a Firebug tab   Firebug itself is also critical for debugging the Javascript side of any app you develop    Assuming you have some JS code of course    I also liked django-viewtools for debugging views interactively using pdb  but I don t use it that much   There are more useful tools like dozer for tracking down memory leaks  there are also other good suggestions given in answers here on SO for memory tracking

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There are a bunch of ways to do it  but the most straightforward is to simply  use the Python debugger  Just add following line in to a Django view function   import pdb  pdb set trace     or   breakpoint     from Python3 7   If you try to load that page in your browser  the browser will hang and you get a prompt to carry on debugging on actual executing code   However there are other options  I am not recommending them      return HttpResponse  variable to inspect      print  variable to inspect     raise Exception  variable to inspect     But the Python Debugger  pdb  is highly recommended for all types of Python code  If you are already into pdb  you d also want to have a look at IPDB that uses ipython for debugging   Some more useful extension to pdb are  pdb    suggested by Antash   pudb  suggested by PatDuJour   Using the Python debugger in Django  suggested by Seafangs

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I really like Werkzeug s interactive debugger  It s similar to Django s debug page  except that you get an interactive shell on every level of the traceback  If you use the django-extensions  you get a runserver plus managment command which starts the development server and gives you Werkzeug s debugger on exceptions   Of course  you should only run this locally  as it gives anyone with a browser the rights to execute arbitrary python code in the context of the server

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During development  adding a quick  assert False  value   can help diagnose problems in views or anywhere else  without the need to use a debugger

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use pdb or ipdb  Diffrence between these two is ipdb supports auto complete   for pdb  import pdb pdb set trace     for ipdb  import ipdb ipdb set trace     For executing new line hit n key  for continue hit c key  check more options by using help pdb

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Add import pdb  pdb set trace   or breakpoint    form python3 7  at the corresponding line in the Python code and execute it  The execution will stop with an interactive shell  In the shell you can execute Python code  i e  print variables  or use commands such as    c continue execution n step to the next line within the same function s step to the next line in this function or a called function q quit the debugger execution   Also see  https   poweruser blog setting-a-breakpoint-in-python-438e23fe6b28

User · Answer

One of your best option to debug Django code is via wdb   https   github com Kozea wdb     wdb works with python 2  2 6  2 7   python 3  3 2  3 3  3 4  3 5  and pypy  Even better  it is possible to debug a python 2 program with a wdb server running on python 3 and vice-versa or debug a program running on a computer with a debugging server running on another computer inside a web page on a third computer    Even betterer  it is now possible to pause a currently running python process thread using code injection from the web interface   This requires gdb and ptrace enabled    In other words it s a very enhanced version of pdb directly in your browser with nice features    Install and run the server  and in your code add   import wdb wdb set trace     According to the author  main differences with respect to pdb are      For those who don   t know the project  wdb is a python debugger like pdb  but with a slick web front-end and a lot of additional features  such as          Source syntax highlighting    Visual breakpoints    Interactive code completion using jedi    Persistent breakpoints    Deep objects inspection using mouse Multithreading   Multiprocessing support   Remote debugging   Watch expressions    In debugger code edition    Popular web servers integration to break on error    In exception breaking during trace  not post-mortem  in contrary to the werkzeug debugger for instance   Breaking in currently running programs through code injection  on supported systems       It has a great browser-based user interface  A joy to use

User · Answer

I use PyCharm and different debug tools  Also have a nice articles set about easy set up those things for novices  You may start here  It tells about PDB and GUI debugging in general with Django projects  Hope someone would benefit from them

User · Answer

I use pyDev with Eclipse really good  set break points  step into code  view values on any objects and variables  try it

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From my own experience   there are two way    use ipdb which is a enhanced debugger likes pdb   import ipdb ipdb set trace   or breakpoint    from python3 7  use django shell  just use the command below  This is very helpfull when you are developing a new view   python manage py shell

User · Answer

From my perspective  we could break down common code debugging tasks into three distinct usage patterns    Something has raised an exception  runserver plus  Werkzeug debugger to the rescue  The ability to run custom code at all the trace levels is a killer  And if you re completely stuck  you can create a Gist to share with just a click  Page is rendered  but the result is wrong  again  Werkzeug rocks  To make a breakpoint in code  just type assert False in the place you want to stop at  Code works wrong  but the quick look doesn t help  Most probably  an algorithmic problem  Sigh  Then I usually fire up a console debugger PuDB  import pudb  pudb set trace    The main advantage over  i pdb is that PuDB  while looking as you re in 80 s  makes setting custom watch expressions a breeze  And debugging a bunch of nested loops is much simpler with a GUI    Ah  yes  the templates  woes  The most common  to me and my colleagues  problem is a wrong context  either you don t have a variable  or your variable doesn t have some attribute  If you re using debug toolbar  just inspect the context at the  Templates  section  or  if it s not sufficient  set a break in your views  code just after your context is filled up   So it goes

User · Answer

A little quickie for template tags    register filter  def pdb element       import pdb  pdb set trace       return element   Now  inside a template you can do    template var pdb    and enter a pdb session  given you re running the local devel server  where you can inspect element to your heart s content   It s a very nice way to see what s happened to your object when it arrives at the template

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I use PyCharm  same pydev engine as eclipse    Really helps me to visually be able to step through my code and see what is happening

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If using Aptana for django development  watch this  http   www youtube com watch v qQh-UQFltJQ  If not  consider using it

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An additional suggestion     You can leverage nosetests and pdb together  rather injecting pdb set trace   in your views manually   The advantage is that you can observe error conditions when they first start  potentially in 3rd party code   Here s an error for me today   TypeError at  db hcm91dmo catalog records   render option   argument after   must be a sequence  not int         Error during template rendering  In template  opt local Library Frameworks Python framework Versions 2 7 lib python2 7 site-packages crispy forms templates bootstrap3 field html  error at line 28 render option   argument after   must be a sequence  not int 18   19             if field is checkboxselectmultiple    20                 include  bootstrap3 layout checkboxselectmultiple html     21             endif    22   23             if field is radioselect    24                 include  bootstrap3 layout radioselect html     25             endif    26   27             if not field is checkboxselectmultiple and not field is radioselect    28             if field is checkbox and form show labels      Now  I know this means that I goofed the constructor for the form  and I even have good idea of which field is a problem   But  can I use pdb to see what crispy forms is complaining about  within a template   Yes  I can   Using the --pdb option on nosetests   tests  nosetests test urls catalog py --pdb  As soon as I hit any exception  including ones handled gracefully   pdb stops where it happens and I can look around     File   opt local Library Frameworks Python framework Versions 2 7 lib python2 7 site-packages django forms forms py   line 537  in   str       return self as widget     File   opt local Library Frameworks Python framework Versions 2 7 lib python2 7 site-packages django forms forms py   line 593  in as widget     return force text widget render name  self value    attrs attrs     File   opt local Library Frameworks Python framework Versions 2 7 lib python2 7 site-packages django forms widgets py   line 513  in render     options   self render options choices   value     File   opt local Library Frameworks Python framework Versions 2 7 lib python2 7 site-packages django forms widgets py   line 543  in render options     output append self render option selected choices   option   TypeError  render option   argument after   must be a sequence  not int INFO lib capture middleware log write to index http   localhost 8082 db hcm91dmo catalog records html  INFO lib capture middleware log write to index end  gt   opt local Library Frameworks Python framework Versions 2 7 lib python2 7 site-packages django forms widgets py 543 render options   - gt  output append self render option selected choices   option    Pdb  import pprint  Pdb  pprint PrettyPrinter indent 4  pprint self   lt django forms widgets Select object at 0x115fe7d10 gt   Pdb  pprint PrettyPrinter indent 4  pprint vars self        attrs        class    select form-control         choices            any type     7   7   type 7    RECTYPE TABLE            is required   False   Pdb             Now  it s clear that my choices argument to the crispy field constructor was as it was a list within a list  rather than a list tuple of tuples     choices            any type     7   7   type 7    RECTYPE TABLE        The neat thing is that this pdb is taking place within crispy s code  not mine and I didn t need to insert it manually

User · Answer

I highly recommend epdb  Extended Python Debugger      https   bitbucket org dugan epdb  One thing I love about epdb for debugging Django or other Python webservers is the epdb serve   command   This sets a trace and serves this on a local port that you can connect to   Typical use case   I have a view that I want to go through step-by-step  I ll insert the following at the point I want to set the trace   import epdb  epdb serve     Once this code gets executed  I open a Python interpreter and connect to the serving instance   I can analyze all the values and step through the code using the standard pdb commands like n  s  etc   In  2   import epdb  epdb connect    Epdb  request  lt WSGIRequest path  foo  GET  lt QueryDict     gt    POST  lt QuestDict     gt        gt   Epdb  request session session key  i31kq7lljj3up5v7hbw9cff0rga2vlq5   Epdb  list  85         raise some error CustomError    86   87       Example login view  88     def login request  username  password    89         import epdb  epdb serve    90  - gt      return my login method username  password   91  92       Example view to show session key  93     def get session key request    94         return request session session key  95   And tons more that you can learn about typing epdb help at any time   If you want to serve or connect to multiple epdb instances at the same time  you can specify the port to listen on  default is 8080    I e   import epdb  epdb serve 4242    gt  gt  import epdb  epdb connect host  192 168 3 2   port 4242    host defaults to  localhost  if not specified   I threw it in here to demonstrate how you can use this to debug something other than a local instance  like a development server on your local LAN   Obviously  if you do this be careful that the set trace never makes it onto your production server   As a quick note  you can still do the same thing as the accepted answer with epdb  import epdb  epdb set trace    but I wanted to highlight the serve functionality since I ve found it so useful

User · Answer

I ve pushed django-pdb to PyPI  It s a simple app that means you don t need to edit your source code every time you want to break into pdb   Installation is just      pip install django-pdb Add  django pdb  to your INSTALLED APPS   You can now run  manage py runserver --pdb to break into pdb at the start of every view     bash  manage py runserver --pdb Validating models     0 errors found Django version 1 3  using settings  testproject settings  Development server is running at http   127 0 0 1 8000  Quit the server with CONTROL-C   GET   function  myview  in testapp views py 6 args     kwargs       gt   Users tom github django-pdb testproject testapp views py 7 myview   - gt  a   1  Pdb    And run  manage py test --pdb to break into pdb on test failures errors     bash  manage py test testapp --pdb Creating test database for alias  default     E                                                                         gt  gt  gt  test error  testapp tests SimpleTest  ---------------------------------------------------------------------- Traceback  most recent call last     File      django-pdb testproject testapp tests py   line 16  in test error     one plus one   four NameError  global name  four  is not defined                                                                          gt   Users tom github django-pdb testproject testapp tests py 16 test error   - gt  one plus one   four  Pdb    The project s hosted on GitHub  contributions are welcome of course

User · Answer

Most options are alredy mentioned  To print template context  I ve created a simple library for that  See https   github com edoburu django-debugtools  You can use it to print template context without any    load    construct      print var      prints variable    print          prints all   It uses a customized pprint format to display the variables in a  lt pre gt  tag

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I just found wdb  http   www rkblog rk edu pl w p debugging-python-code-browser-wdb-debugger  goback  2Egde 25827 member 255996401    It has a pretty nice user interface   GUI with all the bells and whistles   Author says this about wdb -   There are IDEs like PyCharm that have their own debuggers  They offer similar or equal set of features      However to use them you have to use those specific IDEs  and some of then are non-free or may not be available for all platforms   Pick the right tool for your needs    Thought i d just pass it on   Also a very helpful article about python debuggers  https   zapier com engineering debugging-python-boss   Finally  if you d like to see a nice graphical printout of your call stack in Django  checkout  https   github com joerick pyinstrument   Just add pyinstrument middleware ProfilerMiddleware to MIDDLEWARE CLASSES  then add  profile to the end of the request URL to activate the profiler    Can also run pyinstrument from command line or by importing as a module

User · Answer

As mentioned in other posts here - setting breakpoints in your code and walking thru the code to see if it behaves as you expected is a great way to learn something like Django until you have a good sense of how it all behaves - and what your code is doing   To do this I would recommend using WingIde  Just like other mentioned IDEs nice and easy to use  nice layout and also easy to set breakpoints evaluate   modify the stack etc  Perfect for visualizing what your code is doing as you step through it  I m a big fan of it   Also I use PyCharm - it has excellent static code analysis and can help sometimes spot problems before you realize they are there   As mentioned already django-debug-toolbar is essential - https   github com django-debug-toolbar django-debug-toolbar  And while not explicitly a debug or analysis tool - one of my favorites is SQL Printing Middleware available from Django Snippets at https   djangosnippets org snippets 290   This will display the SQL queries that your view has generated  This will give you a good sense of what the ORM is doing and if your queries are efficient or you need to rework your code  or add caching    I find it invaluable for keeping an eye on query performance while developing and debugging my application     Just one other tip - I modified it slightly for my own use to only show the summary and not the SQL statement     So I always use it while developing and testing  I also added that if the len connection queries  is greater than a pre-defined threshold it displays an extra warning   Then if I spot something bad  from a performance or number of queries perspective  is happening I turn back on the full display of the SQL statements to see exactly what is going on   Very handy when you are working on a large Django project with multiple developers

User · Answer

For those that can accidentally add pdb into live commits  I can suggest this extension of  Koobz answer    register filter  def pdb element       from django conf import settings     if settings DEBUG              import pdb         pdb set trace       return element

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Almost everything has been mentioned so far  so I ll only add that instead of pdb set trace   one can use ipdb set trace   which uses iPython and therefore is more powerful  autocomplete and other goodies   This requires ipdb package  so you only need to pip install ipdb

User · Answer

The easiest way to debug python - especially for programmers that are used to Visual Studio - is using PTVS  Python Tools for Visual Studio    The steps are simple     Download and install it from http   pytools codeplex com   Set breakpoints and press F5   Your breakpoint is hit  you can view change the variables as easy as debugging C  C   programs   That s all       If you want to debug Django using PTVS  you need to do the following     In Project settings - General tab  set  Startup File  to  manage py   the entry point of the Django program   In Project settings - Debug tab  set  Script Arguments  to  runserver --noreload   The key point is the  --noreload  here  If you don t set it  your breakpoints won t be hit   Enjoy it

User · Answer

I use PyCharm and stand by it all the way  It cost me a little but I have to say the advantage that I get out of it is priceless  I tried debugging from console and I do give people a lot of credit who can do that  but for me being able to visually debug my application s  is great   I have to say though  PyCharm does take a lot of memory   But then again  nothing good is free in life   They just came with their latest version 3   It also plays very well with Django  Flask and Google AppEngine   So  all in all  I d say it s a great handy tool to have for any developer   If you are not using it yet  I d recommend to get the trial version for 30 days to take a look at the power of PyCharm   I m sure there are other tools also available  such as Aptana   But I guess I just also like the way PyCharm looks   I feel very comfortable debugging my apps there

User · Answer

I find Visual Studio Code is awesome for debugging Django apps  The standard python launch json parameters run python manage py with the debugger attached  so you can set breakpoints and step through your code as you like

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