By using Python Interactive Debugger 'pdb'

First step is to make the Python interpreter enter into the debugging mode.

A. From the Command Line

Most straight forward way, running from command line, of python interpreter

$ python -m pdb scriptName.py
> .../pdb_script.py(7)<module>()
-> """
(Pdb)

B. Within the Interpreter

While developing early versions of modules and to experiment it more iteratively.

$ python
Python 2.7 (r27:82508, Jul  3 2010, 21:12:11)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pdb_script
>>> import pdb
>>> pdb.run('pdb_script.MyObj(5).go()')
> <string>(1)<module>()
(Pdb)

C. From Within Your Program

For a big project and long-running module, can start the debugging from inside the program using import pdb and set_trace() like this:

#!/usr/bin/env python
# encoding: utf-8
#

import pdb

class MyObj(object):
    count = 5
    def __init__(self):
        self.count= 9

    def go(self):
        for i in range(self.count):
            pdb.set_trace()
            print i
        return

if __name__ == '__main__':
    MyObj(5).go()

Step-by-Step debugging to go into more internal

1. Execute the next statement… with “n” (next)

2. Repeating the last debugging command… with ENTER

3. Quitting it all… with “q” (quit)

4. Printing the value of variables… with “p” (print)

   a) p a

5. Turning off the (Pdb) prompt… with “c” (continue)

6. Seeing where you are… with “l” (list)

7. Stepping into subroutines… with “s” (step into)

8. Continuing… but just to the end of the current subroutine… with “r” (return)

9. Assign a new value

   a) !b = "B"

10. Set a breakpoint

    a) break linenumber

    b) break functionname

    c) break filename:linenumber

11. Temporary breakpoint

    a) tbreak linenumber

12. Conditional breakpoint

    a) break linenumber, condition

Note: All these commands should be executed from pdb

For in-depth knowledge, refer:

• https://pymotw.com/2/pdb/

• https://pythonconquerstheuniverse.wordpress.com/2009/09/10/debugging-in-python/