The python debugger pdb

and its friends

All say: "Hello David!"

How do we tackle bugs?

How do we tackle bugs?

  • Intuitive solution: Misuse print() statements for debugging. Better in the long term: Add proper logging to your application!

  • Ask questions: "What's hiding? Where does the bug occur? Is the bug new or old? What has changed? Did i make a bad assumption?"
  • Ask yourself: Do i really understand the code¬†where the error occurs?
  • Do write unit tests for every non-trivial bug you encounter
  • Read your version control history and do diffs
  • Something to add?

What can we do with pdb?

  • Interactive debugging
  • Step through a program at runtime
  • Stop code execution
  • Break on your (conditional) breakpoints
  • Modify and evaluate parts again and again
  • Change stuff in memory
  • Waste a lot of time if we don't have a
    clue about what you're looking for!
  • A debugger is your last resort
    ... or at least never your first

What can we do with pdb?

PDB and friends

Shell:  $> python -m pdb <>
Inline: import pdb; pdb.set_trace()

at your service since Python 1.4 (1996)

  • Get built-in help (help and ?)
  • Show current context (list or list 3, 9 or longlist)
  • Run script until next breakpoint (continue)
  • Repeat last command (press return)
  • Write some python code: for i in range 10: print(i)
  • Print a value (print i or pp for pretty printing)
  • Go to next line in current function (next)
  • Step into a function (step)
  • Print function parameters (args)
  • Step outside of a loop (until)
  • Skip to end of function (return)
  • Set break point (break) and remove them (clear)
    • Set a conditional breakpoint ( `b <line>, a == 2017` )
  • Navigating the execution stack (where, up, down)
  • Start an interactive interpreter (interact, python3 only)
  • Quit (quit)


  • pdbpp - drop in replacement for pdb (cake icing)
  • ipdb - same but with fancy output and completion
  • pudb - A full-screen, console-based Python debugger.
  • pyringe - Debugger capable of attaching to and injecting code into Python processes.
  • wdb - An improbable web debugger through WebSockets.
  • winpdb - A Python Debugger with GUI, capable of remote debugging based on rpdb2.
  • PyCharm - unified debugging with comfortable GUI
    • built-in watcher for variable change like pdbpp

pdb's friends

Back to David's tweet

It would seem that David and other renowned people from the community do not use/like/need debuggers: Your mileage may vary.

The python debugger pdb and its friends

By Stefan Antoni

The python debugger pdb and its friends

Beyond print()ing and log()ging

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