debug ipython notebook with breakpoint

http://www.cs.ucl.ac.uk/scipython/notebooks/02-BreakPoints.html

Debugging

Debugging is the process of finding errors in a program
very important part of programming
can take considerible time
mastering debugging techniques can save you a lot of time

Literature

Python docs on errors and exceptions

http://docs.python.org/2/tutorial/errors.html

Overview

Introduction/ Motivation
Debugging in with IPython

%debug

Exercise
Debugging with break-points
Exercise
Debugging with the logger module
Exercise
Nose tests
Exercise

Debugging with break-points

Break points are a traditional method of inspecting how a program works. Break points are points in a program where the debugger stops each time the program flow passes these points. If
the program is not run with a debugger, break-points are ignored. It can be used to understand how data is changed as the program runs. It is an essential tool in compiled languages, because you cannot manipulate data live in these languages, only view them.
In many programming languages break-points are handled by the IDE and a single click next to the line number in the editor switches them on or off.

ipdb

module

In the following examples I am going to use the Ipython debugging module, which wraps the orignal

pdb

commands
(i.e. uses the same commands as the standard

pdb

). To install

ipdb

just
type

pip install ipdb

in your terminal. If that fails for some reason, you can change the

import

line
below to

import pdb

And you should be able to get the same things done. However, the

ipdb

provides
nice syntax highlighting and generally a nicer view of the stack trace.

set_trace()

In python, break-points are defined with the

set_trace()

function:

Unfortunately the ipdb is broken in the IPython notebook at the moment. Therefore I am showing you this in the terminal. The following code is saved out as

scripts/01.py

.
Open a terminal and run it as

python scripts/01.py

.

# scripts/01.py
import ipdb as pdb

def functionWithBreakpoint():
number = 4
number += 3
pdb.set_trace()
number += 2
return number

print functionWithBreakpoint()

We can play a little bit around with it and try out the commands we know already from the

%debug

magic
we introduced before.

Commands

(courtesy of http://georgejhunt.com/olpc/pydebug/pydebug/ipdb.html)

s(tep) -- Execute the current line, stop at the first possible occasion (either in a function that is called or in the current function).

n(ext) -- Continue execution until the next line in the current function is reached or it returns.

unt(il) -- Continue execution until the line with a number greater than the current one is reached or until the current frame returns.

r(eturn) -- Continue execution until the current function returns.

c(ont(inue)) -- Continue execution, only stop when a breakpoint is encountered.

Stepping though several break-points

# scripts/02.py
import ipdb as pdb

def functionWithBreakpoint():
number = 0
for i in range(10):
number += 2
pdb.set_trace()

numnber -= 1
pdb.set_trace()

return number

print functionWithBreakpoint()