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/

To debug a python script wit input arguments in Spyder IDE (2.3.4)

1. Run > Configure...
2. Select a run configuration > (Choose the script of interest that is open)
3. General settings> Command line options: arg1 arg2 arg3 (use a space as delimiter just as in commandline)
4. Working directory: (Choose the folder)
5. Click OK

Then Debug from menu. This is equivalent to execute the following in iPython console in Spyder.

debugfile('/Users/xxx/xxx/test.py', args='arg1 arg2', wdir='/Users/xxx/xxx/')

Doing it with PyCharm is quite similar.

1. Run > Edit Configurations
2. Choose the python script from the menu
3. The Configuration pane
4. Script parameters: arg1 arg2

Then Run > Debug > Choose the file.

In iPyhton Console you can also try this (suppose test.py is in your current folder):

%run -d test.py arg1 arg2

Python 3.7 adds that feature

From the docs, it looks that your command:

python -m pdb -m my_module

will start working on Python 3.7:

New in version 3.7: pdb.py now accepts a -m option that execute modules similar to the way python3 -m does. As with a script, the debugger will pause execution just before the first line of the module.

Tested on Ubuntu 23.10, Python 3.11.2:

main.py

a = 1
print(a)
a = 2
print(a)

then:

python -m pdb -m main

leads to:

> /home/ciro/test/main.py(1)<module>()
-> a = 1
(Pdb)