Python's built-in debugger, pdb, is a powerful tool that allows you to inspect and control the execution of your Python programs. It helps you identify and fix bugs by providing an interactive environment where you can set breakpoints, step through code, and examine variables.

1. Breakpoints: Points in the code where the execution will pause, allowing you to inspect the state of the program.
2. Stepping: Executing the program one line at a time to closely monitor its behavior.
3. Inspecting Variables: Viewing the values of variables at different points in the program.
4. Call Stack: Understanding the sequence of function calls that led to the current point in the program.

Here are some essential pdb commands:

Let's walk through a practical example to demonstrate how to use pdb for debugging.

Consider the following Python script that calculates the factorial of a number:

1. Import pdb and Set a Breakpoint: Insert the following line before the function call to start the debugger:

   import pdb; pdb.set_trace()
   

   The modified script looks like this:

   def factorial(n):
       if n == 0:
           return 1
       else:
           return n * factorial(n - 1)
   
   if __name__ == "__main__":
       import pdb; pdb.set_trace()
       number = 5
       result = factorial(number)
       print(f"The factorial of {number} is {result}")
   

2. Run the Script: Execute the script in the terminal:

   python script.py
   

3. Interactive Debugging: The execution will pause at the pdb.set_trace() line, and you will enter the pdb interactive mode. You can now use pdb commands to debug the script.

   • Set a Breakpoint:

     (Pdb) b factorial
     

     This sets a breakpoint at the start of the factorial function.

   • Continue Execution:

     (Pdb) c
     

     The program will run until it hits the breakpoint at the factorial function.

   • Step Through Code:

     (Pdb) s
     

     Step into the factorial function.

   • Inspect Variables:

     (Pdb) p n
     

     Print the value of n.

   • Continue Execution:

     (Pdb) c
     

     Continue execution until the next breakpoint or the end of the program.

Modify the following script to use pdb for debugging. Set breakpoints and step through the code to understand its behavior.

1. Import pdb and Set a Breakpoint:

   def sum_of_squares(n):
       total = 0
       for i in range(1, n + 1):
           total += i * i
       return total
   
   if __name__ == "__main__":
       import pdb; pdb.set_trace()
       number = 5
       result = sum_of_squares(number)
       print(f"The sum of squares up to {number} is {result}")
   

2. Run the Script:

   python script.py
   

3. Interactive Debugging:

   • Set a Breakpoint:

     (Pdb) b sum_of_squares
     

   • Continue Execution:

     (Pdb) c
     

   • Step Through Code:

     (Pdb) s
     

   • Inspect Variables:

     (Pdb) p n
     (Pdb) p total
     

Using pdb for debugging allows you to interactively inspect and control the execution of your Python programs. By setting breakpoints, stepping through code, and inspecting variables, you can identify and fix bugs more efficiently. Practice using pdb with different scripts to become proficient in debugging Python code.