Debugging

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How can I debug my program?

Objectives

• Debug code containing an error systematically.

• Identify ways of making code less error-prone and more easily tested.

• Know of the ways to debug using Python and IPython debugging tools in Pycharm, qtconsole, and the ipython console

Pdb and ipdb

• The python debugger is a command line debugger that can be invoked from a system terminal using the command python -m pdb myscript.py.
• ipdb provides additional functionality.

Debugging in jupyter qtconsole

When we have an error in a script while using the run magic in qtconsole we do not see any output. This is an known error in qtconsole. Two options exist at this point.

We can run the script in debugger mode using the -d flag. This will allow us to exam the variables as we execute the script. We will see the exception raised by our assert statement in this case. The other option is to use the ipython console. This provides a similar interface to ipython but does not support inline graphics.

In addition to this bug the qtconsole does not have readline support for the debugger. This means that we must type commands ourselves without relying on the arrow keys to recover previous input.

ipdb reference

This is helpful. And scipy lectures

# In order to debug a function in qtconsole:
from IPython.core.debugger import Pdb; ipdb=Pdb()
ipdb.runcall(foo, 1, 2) Found [here](http://stackoverflow.com/questions/12646670/stepping-into-a-function-in-ipython):

After a runtime error use the debug magic to inspect the variables when execution failed:

%debug

Finally, run a file in debug mode using the -d flag to the run magic:

%run -d metasearch_analysis.py

—-Getting help —-

# display commands:
? 
# display help for w:
?w 

—-Getting Context—-

# where in the current call stack:
w 
# long list of code surrounding current location of execution:
ll  
# list shorter context:
l
# list local variables:
dir()
# list arguments to current function:
a
# print variable X
p X
# pretty print variable X
pp X

—- Controlling execution —-

# continue until the next breakpoint:
c 
# execute the next line:
n 
# execute the current line. stop at the first occasion
# 
# temporary  breakpoint here:
tbreak 5 
# no arguments lists them:
break 
# breakpoint at line 5:
b 5 
# clear breakpoint number 2:
cl 2 
# ignore breakpoint 1 for the next 3 crossings:
ignore 1 3 

Key Points