let's use pdb

# ...
import pdb; pdb.set_trace()
# ...

the basics

let's use pdb

l (list)

list source code for the current file.

ll (longlist)

list all source code for the current function or frame.

New in version 3.2

show me where I am

let's use pdb

moving through the code

n (next)

continue until the next line in the current function

s (step)

stop at the first possible occasion

r (return)

continue until the current function returns

c (continue)

continue execution, only stop at a breakpoint

let's use pdb

$ python -m pdb you_script.py

another way to use pdb

let's use pdb

break and inspect

b (break)

set a breaking point in the source

a (args)

print the argument list of the current function

p (print) pp (pretty print) 

print the evaluated expression in the current context

let's use pdb

run commands everytime

.pdbrc

let's use pdb

run commands everytime

display

display the value each time it changes

interact

start an interactive interpreter