Using ipdb to Debug Python Code

Interactive Python Debugger(IPDB) is a powerful debugging tool that is built on top of the IPython shell. It allows developers to step through their code line by line, set breakpoints, and inspect variables in real-time. Unlike other debuggers, IPDB runs inside the Python interpreter, which makes it easier to use and integrate with other tools.

Required Modules

pip install ipdb

How to use IPDB

After the debugger is installed, now you can start using IPDB by adding the following line of code:

import ipdb; 
#code
ipdb.set_trace()

This line of code will set a breakpoint in your code at the point where it is executed. When the code reaches this point, the debugger will pause execution and drop you into an interactive shell where you can start debugging.

From here, you can use various IPDB commands to step through your code, inspect variables, and more. Some of the most commonly used commands include:

Examples

Iterating through the loop

The function factorial(n), which is defined in this code, uses recursion to get the factorial of a given integer n. In this case, the method returns 1 if n is less than or equal to 0. If not, it calls itself recursively with n-1 as the argument and multiplies the outcome by n. After the factorial() function declaration, the code additionally places a breakpoint using ipdb.set_trace().

import ipdb

def factorial(n):
    if n <= 0:
        return 1
    else:
        return n * factorial(n-1)

ipdb.set_trace()

num = 5
fact = factorial(num)

print("Factorial of", num, "is", fact)

Output :

Viewing sorting program

Sort_numbers() and main() are two functions defined in this code. sort_numbers() takes a list of numbers as input, uses ipdb.set_trace() to set a breakpoint, uses the sorted() function to sort the numbers in descending order, and then returns the sorted list.

import ipdb

def sort_numbers(numbers):
    ipdb.set_trace()
    sorted_numbers = sorted(numbers, reverse=True)
    return sorted_numbers


def main():
    numbers = [5, 2, 8, 1, 9, 4]
    sorted_numbers = sort_numbers(numbers)
    print(sorted_numbers)

main()

Output:

When we run the code again, we'll see that it pauses at the breakpoint, and we can use various IPDB commands to examine the state of the program. For example, we can use the print command to check the value of the numbers argument :

Simple multiply and add the program

In this code, we are using the ipdb.set_trace() instruction, the code places a breakpoint. When this happens, the Python debugger will halt the program's execution at that particular moment to allow the user to investigate the status of the program at that time. Following the breakpoint, the program executes add(), passing result1 and y as inputs, and saving the outcome in result2. The value of result2 is finally written to the console.

import ipdb

def multiply(x, y):
    result = x * y
    return result

def add(x, y):
    result = x + y
    return result

def main():
    x = 5
    y = 10
    result1 = multiply(x, y)
    ipdb.set_trace()   #breakpoint created by ipdb
    result2 = add(result1, y)
    print(result2)

main()

Output :

Difference between pdb debugger and ipdb debugger

Both PDB and IPDB support column-based debugging, which means that you can set a breakpoint at a specific column in a line of code. However, IPDB offers some additional functionality that makes it more user-friendly and powerful than PDB.