How to step through Python code to help debug issues

Question

In Java C  you can easily step through code to trace what might be going wrong  and IDE s make this process very user friendly   Can you trace through python code in a similar fashion

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Starting in Python 3 7  you can use the breakpoint   built-in function to enter the debugger    foo   breakpoint      drop into the debugger at this point bar     By default  breakpoint   will import pdb and call pdb set trace    However  you can control debugging behavior via sys breakpointhook   and use of the environment variable PYTHONBREAKPOINT   See PEP 553 for more information

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https   wiki python org moin PythonDebuggingTools  pudb is a good drop-in replacement for pdb

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Visual Studio with PTVS could be an option for you  http   www hanselman com blog OneOfMicrosoftsBestKeptSecretsPythonToolsForVisualStudioPTVS aspx

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ipdb  IPython debugger   ipdb adds IPython functionality to pdb  offering the following HUGE improvements    tab completion show more context lines syntax highlight   Much like pdg  ipdb is still far from perfect and completely rudimentary if compared to GDB  but it is already a huge improvement over pdb   Usage is analogous to pdb  just install it with   python3 -m pip install --user ipdb   and then add to the line you want to step debug from     import    ipdb   set trace context 21    You likely want to add a shortcut for that from your editor  e g  for Vim snipmate I have   snippet ipd       import    ipdb   set trace context 21    so I can type just ipd lt tab gt  and it expands to the breakpoint  Then removing it is easy with dd since everything is contained in a single line   context 21 increases the number of context lines as explained at  How can I make ipdb show more lines of context while debugging   Alternatively  you can also debug programs from the start with   ipdb3 main py   but you generally don t want to do that because    you would have to go through all function and class definitions as Python reads those lines I don t know how to set the context size there without hacking ipdb  Patch to allow it  https   github com gotcha ipdb pull 155   Or alternatively  as in raw pdb 3 2  you can set some breakpoints from the command line   ipdb3 -c  b 12  -c  b myfunc    test a py   although -c c is broken for some reason  https   github com gotcha ipdb issues 156  python -m module debugging has been asked at  How to debug a Python module run with python -m from the command line  and since Python 3 7 can be done with   python -m pdb -m my module   Serious missing features of both pdb and ipdb compared to GDB    persistent command history across sessions  Save command history in pdb   ipdb specific annoyances    multithreading does not work well if you don t hack some settings      ipdb  multiple threads and autoreloading programs causing ProgrammingError https   github com gotcha ipdb issues 51    Tested in Ubuntu 16 04  ipdb  0 11  Python 3 5 2

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There is a module called  pdb  in python  At the top of your python script you do  import pdb pdb set trace     and you will enter into debugging mode  You can use  s  to step   n  to follow next line similar to what you would do with  gdb  debugger

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There exist breakpoint   method nowadays  which replaces import pdb  pdb set trace     It also has several new features  such as possible environment variables

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Yes  There s a Python debugger called pdb just for doing that   You can launch a Python program through pdb by using pdb myscript py or python -m pdb myscript py   There are a few commands you can then issue  which are documented on the pdb page   Some useful ones to remember are    b  set a breakpoint c  continue debugging until you hit a breakpoint s  step through the code n  to go to next line of code l  list source code for the current file  default  11 lines including the line being executed  u  navigate up a stack frame d  navigate down a stack frame p  to print the value of an expression in the current context   If you don t want to use a command line debugger  some IDEs like Pydev  Wing IDE or PyCharm have a GUI debugger  Wing and PyCharm are commercial products  but Wing has a free  Personal  edition  and PyCharm has a free community edition

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If you come from Java C  background I guess your best bet would be to use Eclipse with Pydev  This gives you a fully functional IDE with debugger built in  I use it with django as well

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Programmatically stepping and tracing through python code is possible too  and its easy    Look at the sys settrace   documentation for more details  Also here is a tutorial to get you started

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PyCharm is an IDE for Python that includes a debugger  Watch this YouTube video for an introduction on using PyCharm s debugger to step through code   PyCharm Tutorial - Debug python code using PyCharm  Note  This is not intended to be an endorsement or review  PyCharm is a commercial product that one needs to pay for  but the company does provide a free license to students and teachers  as well as a  lightweight  Community version that is free and open-source

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If you want an IDE with integrated debugger  try PyScripter

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By using Python Interactive Debugger  pdb   First step is to make the Python interpreter to enter into the debugging mode   A  From the Command Line  Most straight forward way  running from command line  of python interpreter    python -m pdb scriptName py  gt      pdb script py 7  lt module gt    - gt       Pdb    B  Within the Interpreter  While developing early versions of modules and to experiment it more iteratively     python Python 2 7  r27 82508  Jul  3 2010  21 12 11   GCC 4 0 1  Apple Inc  build 5493   on darwin Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import pdb script  gt  gt  gt  import pdb  gt  gt  gt  pdb run  pdb script MyObj 5  go      gt   lt string gt  1  lt module gt     Pdb    C  From Within Your Program  For a big project and long-running module  can start the debugging from inside the program using  import pdb and set trace   like this       usr bin env python   encoding  utf-8    import pdb  class MyObj object       count   5     def   init   self           self count  9      def go self           for i in range self count               pdb set trace               print i         return  if   name         main         MyObj 5  go     Step-by-Step debugging to go into more internal   Execute the next statement    with    n     next  Repeating the last debugging command    with ENTER Quitting it all    with    q     quit  Printing the value of variables    with    p     print   a   p a Turning off the  Pdb  prompt    with    c     continue  Seeing where you are    with    l     list  Stepping into subroutines    with    s     step into  Continuing    but just to the end of the current subroutine    with    r     return  Assign a new value  a    b    B  Set a breakpoint  a   break linenumber  b   break functionname  c   break filename linenumber Temporary breakpoint  a   tbreak linenumber Conditional breakpoint  a   break linenumber  condition   Note   All these commands should be execute from   pdb  For in-depth knowledge  refer -  https   pymotw com 2 pdb   https   pythonconquerstheuniverse wordpress com 2009 09 10 debugging-in-python

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Python Tutor is an online single-step debugger meant for novices  You can put in code on the edit page then click  Visualize Execution  to start it running   Among other things  it supports    live help from volunteers  which is great for newbies  setting breakpoints hiding variables saving sharing a few other languages like Java  JS  Ruby  C  C     However it also doesn t support a lot of things  for example    Reading writing files - use io StringIO and io BytesIO instead  demo Code that is too large  runs too long  or defines too many variables or objects Command-line arguments Lots of standard library modules like argparse  csv  enum  html  os  struct  weakref

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Let s take look at what breakpoint   can do for you in 3 7   I have installed ipdb and pdbpp  which are both enhanced debuggers  via pip install pdbpp pip install ipdb  My test script  really doesn t do much  just calls breakpoint     test 188 breakpoint py myvars dict foo  quot bar quot   print  quot before breakpoint   quot   breakpoint        print f quot after breakpoint myvars  myvars  quot      breakpoint   is linked to the PYTHONBREAKPOINT environment variable  CASE 1   disabling breakpoint   You can set the variable via bash as usual export PYTHONBREAKPOINT 0 This turns off breakpoint   where it does nothing  as long as you haven t modified sys breakpointhook   which is outside of the scope of this answer   This is what a run of the program looks like   venv38  myuser explore  export PYTHONBREAKPOINT 0  venv38  myuser explore  python test 188 breakpoint py before breakpoint   after breakpoint myvars   foo    bar    venv38  myuser explore    Didn t stop  because I disabled breakpoint   Something that pdb set trace   can t do   CASE 2   using the default pdb behavior  Now  let s unset PYTHONBREAKPOINT which puts us back to normal  enabled-breakpoint behavior  it s only disabled when 0 not when empty    venv38  myuser explore  unset PYTHONBREAKPOINT  venv38  myuser explore  python test 188 breakpoint py before breakpoint    0   gt   Users myuser kds2 wk explore test 188 breakpoint py 6  lt module gt    - gt  print f quot after breakpoint myvars  myvars  quot    Pdb    print  quot pdbpp replaces pdb because it was installed quot   pdbpp replaces pdb because it was installed  Pdb    c after breakpoint myvars   foo    bar     It stopped  but I actually got pdbpp because it replaces pdb entirely while installed  If I unistalled pdbpp  I d be back to normal pdb  Note   a standard pdb set trace   would still get me pdbpp CASE 3  calling a custom debugger But let s call ipdb instead   This time  instead of setting the environment variable  we can use bash to set it only for this one command   venv38  myuser explore  PYTHONBREAKPOINT ipdb set trace py test 188 breakpoint py before breakpoint    gt   Users myuser kds2 wk explore test 188 breakpoint py 6  lt module gt          5 breakpoint   ---- gt  6 print f quot after breakpoint myvars  myvars  quot         7  ipdb gt  print  quot and now I invoked ipdb instead quot   and now I invoked ipdb instead ipdb gt  c after breakpoint myvars   foo    bar    Essentially  what it does  when looking at  PYTHONBREAKPOINT  from ipdb import set trace    function imported on the right-most     set trace    Again  much cleverer than a plain old pdb set trace    in practice   I d probably settle on a debugger  Say I want ipdb always  I would   export it via  profile or similar  disable on a command by command basis  without modifying the normal value  Example  pytest and debuggers often make for unhappy couples    venv38  myuser explore  export PYTHONBREAKPOINT ipdb set trace  venv38  myuser explore  echo  PYTHONBREAKPOINT ipdb set trace  venv38  myuser explore  PYTHONBREAKPOINT 0 pytest test 188 breakpoint py                                     test session starts                                      platform darwin -- Python 3 8 6  pytest-5 1 2  py-1 9 0  pluggy-0 13 1 rootdir   Users myuser kds2 wk explore plugins  celery-4 4 7  cov-2 10 0 collected 0 items                                     no tests ran in 0 03s                                      venv38  myuser explore  echo  PYTHONBREAKPOINT ipdb set trace  p s  I m using bash under macos  any posix shell will behave substantially the same   Windows  either powershell or DOS  may have different capabilities  especially around PYTHONBREAKPOINT  lt some value gt   lt some command gt  to set a environment variable only for one command

[python] How to step through Python code to help debug issues?

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