How to step through Python code to help debug issues?

Question

Yes! There's a Python debugger called pdb just for doing that!

You can launch a Python program through pdb by using pdb myscript.py or python -m pdb myscript.py.

There are a few commands you can then issue, which are documented on the pdb page.

Some useful ones to remember are:

• b: set a breakpoint
• c: continue debugging until you hit a breakpoint
• s: step through the code
• n: to go to next line of code
• l: list source code for the current file (default: 11 lines including the line being executed)
• u: navigate up a stack frame
• d: navigate down a stack frame
• p: to print the value of an expression in the current context

If you don't want to use a command line debugger, some IDEs like Pydev, Wing IDE or PyCharm have a GUI debugger. Wing and PyCharm are commercial products, but Wing has a free "Personal" edition, and PyCharm has a free community edition.

By using Python Interactive Debugger 'pdb'

First step is to make the Python interpreter to 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 execute from **pdb

For in-depth knowledge, refer:-

https://pymotw.com/2/pdb/

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

There is a module called 'pdb' in python. At the top of your python script you do

import pdb
pdb.set_trace()

and you will enter into debugging mode. You can use 's' to step, 'n' to follow next line similar to what you would do with 'gdb' debugger.

Starting in Python 3.7, you can use the breakpoint() built-in function to enter the debugger:

foo()
breakpoint()  # drop into the debugger at this point
bar()

By default, breakpoint() will import pdb and call pdb.set_trace(). However, you can control debugging behavior via sys.breakpointhook() and use of the environment variable PYTHONBREAKPOINT.

See PEP 553 for more information.

ipdb (IPython debugger)

ipdb adds IPython functionality to pdb, offering the following HUGE improvements:

• tab completion
• show more context lines
• syntax highlight

Much like pdg, ipdb is still far from perfect and completely rudimentary if compared to GDB, but it is already a huge improvement over pdb.

Usage is analogous to pdb, just install it with:

python3 -m pip install --user ipdb

and then add to the line you want to step debug from:

__import__('ipdb').set_trace(context=21)

You likely want to add a shortcut for that from your editor, e.g. for Vim snipmate I have:

snippet ipd
    __import__('ipdb').set_trace(context=21)

so I can type just ipd<tab> and it expands to the breakpoint. Then removing it is easy with dd since everything is contained in a single line.

context=21 increases the number of context lines as explained at: How can I make ipdb show more lines of context while debugging?

Alternatively, you can also debug programs from the start with:

ipdb3 main.py

but you generally don't want to do that because:

• you would have to go through all function and class definitions as Python reads those lines
• I don't know how to set the context size there without hacking ipdb. Patch to allow it: https://github.com/gotcha/ipdb/pull/155

Or alternatively, as in raw pdb 3.2+ you can set some breakpoints from the command line:

ipdb3 -c 'b 12' -c 'b myfunc' ~/test/a.py

although -c c is broken for some reason: https://github.com/gotcha/ipdb/issues/156

python -m module debugging has been asked at: How to debug a Python module run with python -m from the command line? and since Python 3.7 can be done with:

python -m pdb -m my_module

Serious missing features of both pdb and ipdb compared to GDB:

• persistent command history across sessions: Save command history in pdb

ipdb specific annoyances:

• multithreading does not work well if you don't hack some settings...
  • ipdb, multiple threads and autoreloading programs causing ProgrammingError
  • https://github.com/gotcha/ipdb/issues/51

Tested in Ubuntu 16.04, ipdb==0.11, Python 3.5.2.