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: