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Pdb - Python Debugger

The Python Debugger, or pdb, is the easiest way to debug python code from the terminal.

Table of Contents

Using pdb

Breakpoints

To set a breakpoint for pdb, you can either use the breakpoint() builtin function, or use the pdb module.

breakpoint()

import pdb
pdb.set_trace()

Debugging an Entire Program

Alternatively, you can debug a whole program by running it with the flag -m pdb.

python -m pdb exppdb.py

  • c: Continue execution
  • q: Quit the debugger/execution
  • n: Step to next line within the same function
  • : Step to next line in this function or a called function

Commands in Pdb

  • help: To display all commands.
  • w/where: Print a stack trace.
    • An arrow will indicate the "current frame", which determines the context of most commands.
    • bt: An alias for w/where.
  • u/up: Move the one level up in the stack trace to an older frame.
  • d/down: Move down one level in the stack trace to a newer frame.
  • where: Display the stack trace and line number of the current line.
  • n/next: Execute the current line and move to the next line ignoring function calls.
  • s/step: Step into functions called at the current line.
  • whatis var: Show the type of the given variable.
  • j n/jump n: Jump to the given line and execute that line next.
  • unt n/until n: Run until you reach line number n.
  • longlist/ll: List the source code for the current function.
  • alias: Create an alias to use in pdb.
  • interact: Start an interactive interpreter in the current namespace (scope).

Output of the help command

If you run help it'll give you all the commands available: 

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

Getting Help with Specific Commands

You can run help <cmd> for any of the commands to display what the command does, and how to use it. 

(Pdb) help c
c(ont(inue))
        Continue execution, only stop when a breakpoint is encountered.