The Python Standard Library includes an interactive debugging library called pdb. pdb has extensive capabilities, the most commonly used being the ability to ‘step-through’ a program.

To immediately enter into step-through debugging use:

python -m pdb <my_file.py>

This will start the debugger at the first line of the program.

Usually you will want to target a specific section of the code for debugging. To do this we import the pdb library and use set_trace() to interrupt the flow of this troubled example code.

import pdb

def divide(a, b):
    pdb.set_trace()
    return a/b 
    # What's wrong with this? Hint: 2 != 3

print divide(1, 2)

Running this program will launch the interactive debugger.

python foo.py 
> ~/scratch/foo.py(5)divide()
-> return a/b
(Pdb)

Often this command is used on one line so it can be commented out with a single # character

import pdf; pdb.set_trace()

At the (Pdb) prompt commands can be entered. These commands can be debugger commands or python. To print variables we can use p from the debugger, or python’s print.

(Pdb) p a
1
(Pdb) print a
1

To see list of all local variables use

locals

build-in function

These are good debugger commands to know:

b <n> | <f>: set breakpoint at line *n* or function named *f*.
# b 3
# b divide
b: show all breakpoints.
c: continue until the next breakpoint.
s: step through this line (will enter a function).
n: step over this line (jumps over a function).
r: continue until the current function returns.
l: list a window of code around this line.
p <var>: print variable named *var*.
# p x
q: quit debugger.
bt: print the traceback of the current execution call stack
up: move your scope up the function call stack to the caller of the current function
down: Move your scope back down the function call stack one level
step: Run the program until the next line of execution in the program, then return control back to the debugger
next: run the program until the next line of execution in the current function, then return control back to the debugger
return: run the program until the current function returns, then return control back to the debugger
continue: continue running the program until the next breakpoint (or set_trace si called again)

The debugger can also evaluate python interactively:

-> return a/b
(Pdb) p a+b
3
(Pdb) [ str(m) for m in [a,b]] 
['1', '2']
(Pdb) [ d for d in xrange(5)]
[0, 1, 2, 3, 4]

Note: