In-depth PDB

Nathan Yergler

presentotron.com/nyergler/pdb

github.com/nyergler/in-depth-pdb

Better than print statements!

• explore state of a running program, or a dead one
• repeatedly run a program
• build the tools you need for your program

Explicit Trace Points

Invocation

    import pdb; pdb.set_trace()

Navigation in pdb

• next: execute current line
• step: execute current line and stop as soon as possible
• cont: leave the debugger and let your program continue executing
• Enter repeats the previous command

Executing Code Under PDB

Running PDB as a Script

    python -m pdb django runserver

pdb.run

    pdb.run("print('Hey')")
    pdb.runcall(int, '4')

Post-Mortem debugging and More Commands

• Now we have the tools to debug our stuff in interesting ways.
• Example project: HTTP Service for postfix calculations.
• When running under pdb (python -m pdb), you get a post mortem debugger after uncaught exceptions

Nice built-ins for inspection

    (Pdb) args    # prints args to current function
    (Pdb) list    # lists 5 lines before and after current point
    (Pdb) ll      # long listing: just the current function/method
    (Pdb) pp var  # pretty-print var

Evaluate expressions with !

    (Pdb) b+c
    Error: ...no +c on path.  whaaaat?  PDB thinks you want to set a breakpoint at "+c"
    (Pdb) !b+c
    [value]

Navigating the Stack

    (Pdb) where # where am I in the call stack? last line is where we are
    (Pdb) up    # move up the stack
    (Pdb) down  # move down the stack

Post-mortem Debugging

• Effectively acts like a top-level, bare exception handler

    import sys
    def main():
        # stuff
  
    if __name__ == '__main__':
        try:
            sys.exit(main())
        except:
            import pdb; pdb.pm()

Breakpoints

• Way to look at code without editing it; good for third-party stuff

    (Pdb) break pfcalc.rpn_app  # can break at some callable
    (Pdb) break pfcalc.py:41    # ...or a line number
    (Pdb) break                 # show all breakpoints and how many times they've been triggered

• Can make breakpoint conditional: only break if python expression is truthy

    (Pdb) break pfcalc.rpn_app, environ['HTTP_METHOD'] != 'GET'

Extending PDB

Aliases

• Can refer to other aliases
• Defined where???

    alias dr pp dir(%1)
    alias loc locals().keys()
    alias printdict for key, value in %1.items(): print("{0}: {1}".format(key, value)

Breakpoint Commands

• Executed when a breakpoint is hit, just like you typed it yourself
• Anything except next, step, end

    (Pdb) commands 1
    (com) pp self.state
    (com) pp value_or_operator
    (com) continue

.pdbrc Hooray!!!

• Exists home directory version first, then in current dir
• Executed line-by-line in pdb
• Comments allowed, yay!
• Can setup variable watches!
  • google "How do you watch a variable in pdb" on stackoverflow

See also

• ipdb
  • syntax highlighting
  • tab completion
• rdb: PDB over a socket
• pudb: full screen, console debugger
• pdb++
• wdb