practical Python

debugging

kevin@freshbooks.com

agenda

Debugging
Intro to pdb

variable inspection
execution control
stack frame movement
breakpoints
custom aliases

Alternatives to pdb

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it

- Brian W. Kernighan

intro to pdb

Included in the standard lib
Modes of operation

run, script, post-mortem mode
trace mode

All these modes drop you into a pdb prompt
You can

Inspect
Control
Move

INSPECTION COMMANDS

p(rint) - print value of variable
display - "watch" the value of a variable
a(rgs) - arguments of the current function
r(v) - the return value of the last function
l(ist) - where in the source code am I?

Execution control commands

s(tep) - Step into the function
n(ext) - Step over the function
r(eturn) - Return from the function
until - Useful for breaking out of a loop
c(ontinue) - Continue execution

stack Frame control commands

w(here) - where in the stack am I?
u(p) - move up the stack frame
d(own) - move down the stack frame

Custom aliases

Use the `alias` command

```
alias nl n;;l 
```
```
alias sl s;;l 
```

$HOME/.pdbrc

breakpoints

alternatives to pdb

pdb++
pudb
ipdb
debug

pdb++

# installpip install pdbpp# usageimport pdb; pdb.set_trace()

drop-in replacement of pdb
tab-completion
syntax colouring

ipdb

# installpip install ipdb
pip install debug# usageimport ipdb; ipdb.set_trace()# orimport debug  # yeah, that's it!

uses ipython
tab-completion
syntax colouring

pudb

# installpip install pudb# usageimport pudb; pudb.set_trace()

Console "GUI" debugger

uses libncurses

Very handy for navigating unfamiliar code