In this chapter, we will learn how to debug thread applications. We will also learn the importance of debugging.

What is Debugging?

In computer programming, debugging is the process of finding and removing the bugs, errors and abnormalities from computer program. This process starts as soon as the code is written and continues in successive stages as code is combined with other units of programming to form a software product. Debugging is part of the software testing process and is an integral part of the entire software development life cycle.

Python Debugger

The Python debugger or the pdb is part of the Python standard library. It is a good fallback tool for tracking down hard-to-find bugs and allows us to fix faulty code quickly and reliably. Followings are the two most important tasks of the pdp debugger −

• It allows us to check the values of variables at runtime.
• We can step through the code and set breakpoints also.

We can work with pdb in the following two ways −

• Through the command-line; this is also called postmortem debugging.
• By interactively running pdb.

Working with pdb

For working with the Python debugger, we need to use the following code at the location where we want to break into the debugger −

import pdb;
pdb.set_trace()

Consider the following commands to work with pdb through command-line.

• h(help)
• d(down)
• u(up)
• b(break)
• cl(clear)
• l(list))
• n(next))
• c(continue)
• s(step)
• r(return))
• b(break)

Following is a demo of the h(help) command of the Python debugger −

import pdb

pdb.set_trace()
--Call--
>d:programdatalibsite-packagesipythoncoredisplayhook.py(247)__call__()
-> def __call__(self, result = None):
(Pdb) h

Documented commands (type help <topic>):
========================================
EOF   c         d       h        list     q       rv      undisplay
a     cl        debug   help     ll       quit    s       unt
alias clear     disable ignore   longlist r       source  until
args  commands  display interact n        restart step    up
b     condition down    j        next     return  tbreak  w
break cont      enable  jump     p        retval  u       whatis
bt    continue  exit    l        pp       run     unalias where

Miscellaneous help topics:
==========================
exec pdb

Example

While working with Python debugger, we can set the breakpoint anywhere in the script by using the following lines −

import pdb;
pdb.set_trace()

After setting the breakpoint, we can run the script normally. The script will execute until a certain point; until where a line has been set. Consider the following example where we will run the script by using the above-mentioned lines at various places in the script −

import pdb;
a = "aaa"
pdb.set_trace()
b = "bbb"
c = "ccc"
final = a + b + c
print (final)

When the above script is run, it will execute the program till a = “aaa”, we can check this in the following output.

Output

--Return--
> <ipython-input-7-8a7d1b5cc854>(3)<module>()->None
-> pdb.set_trace()
(Pdb) p a
''aaa''
(Pdb) p b
*** NameError: name ''b'' is not defined
(Pdb) p c
*** NameError: name ''c'' is not defined

After using the command ‘p(print)’ in pdb, this script is only printing ‘aaa’. This is followed by an error because we have set the breakpoint till a = “aaa”.

Similarly, we can run the script by changing the breakpoints and see the difference in the output −

import pdb
a = "aaa"
b = "bbb"
c = "ccc"
pdb.set_trace()
final = a + b + c
print (final)

Output

--Return--
> <ipython-input-9-a59ef5caf723>(5)<module>()->None
-> pdb.set_trace()
(Pdb) p a
''aaa''
(Pdb) p b
''bbb''
(Pdb) p c
''ccc''
(Pdb) p final
*** NameError: name ''final'' is not defined
(Pdb) exit

In the following script, we are setting the breakpoint in the last line of the program −

import pdb
a = "aaa"
b = "bbb"
c = "ccc"
final = a + b + c
pdb.set_trace()
print (final)

The output is as follows −

--Return--
> <ipython-input-11-8019b029997d>(6)<module>()->None
-> pdb.set_trace()
(Pdb) p a
''aaa''
(Pdb) p b
''bbb''
(Pdb) p c
''ccc''
(Pdb) p final
''aaabbbccc''
(Pdb)