Debugging in Python

A guide to knowing and implementing the Python debugger module.

The Python module used to debug the code is called as “pdb”. It provides an interactive debugging program for Python programs. It has features that allow you to pause the program, check the values of different variables and watch the step-by-step execution of the program. This helps in finding bugs in the logic.

Let’s consider a code with an error to see how this works.

>>> import pdb
>>> a = [11,44,77]
>>> b = 1
>>> c = 9
>>> result = b + c
>>> print(result)
# wrong line of code
>>> result2 = a + b
>>> print(result2)
10
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-29b30ba110f3> in <module>
      7 
      8 # wrong line of code
----> 9 result2 = a + b
     10 print(result2)
TypeError: can only concatenate list (not "int") to list

So as you can see there is an error because a list and integer cannot be added. Now we will set a trace using the module. So go to the line where there is an error and trace it. When this code is run, the interactive debugging environment is shown. Press ‘q’ to quit the debugging environment.

Source: Author

>>> a = [11,44,77]
>>> b = 1
>>> c = 9
>>> result = b + c
>>> print(result)
>>> pdb.set_trace()
# wrong line of code
>>> result2 = a + b
>>> print(result2)
10
--Return--
None
> <ipython-input-3-c41ee9df98d8>(8)<module>()
      6 print(result)
      7 
----> 8 pdb.set_trace()
      9 
     10 # wrong line of code
ipdb> q
---------------------------------------------------------------------------
BdbQuit                                   Traceback (most recent call last)
<ipython-input-3-c41ee9df98d8> in <module>
      6 print(result)
      7 
----> 8 pdb.set_trace()
      9 
     10 # wrong line of code
D:\Anaconda3\lib\bdb.py in trace_dispatch(self, frame, event, arg)
     90             return self.dispatch_call(frame, arg)
     91         if event == 'return':
---> 92             return self.dispatch_return(frame, arg)
     93         if event == 'exception':
     94             return self.dispatch_exception(frame, arg)
D:\Anaconda3\lib\bdb.py in dispatch_return(self, frame, arg)
    152             finally:
    153                 self.frame_returning = None
--> 154             if self.quitting: raise BdbQuit
    155             # The user issued a 'next' or 'until' command.
    156             if self.stopframe is frame and self.stoplineno != -1:
BdbQuit:

Refer to the notebook here.

Beginner-level books to refer to learn Python:

Python Crash Course, 2nd Edition: A Hands-On, Project-Based Introduction to Programming

Learn Python the Hard Way: A Very Simple Introduction to the Terrifyingly Beautiful World of…

Think Python, 2e: How to Think Like a Computer Scientist

Advance-level books to refer to learn Python:

Programming Python: Powerful Object-Oriented Programming

Advanced Python Programming: Build high performance, concurrent, and multi-threaded apps with…

Reach out to me: LinkedIn

Check out my other work: GitHub