How to Use pdb Debugger in Python: Simple Guide

Use the pdb module in Python by importing it and calling pdb.set_trace() where you want to pause execution. This opens an interactive debugger allowing you to inspect variables and step through code line-by-line.

Syntax

To use the pdb debugger, first import the pdb module. Then insert pdb.set_trace() at the point in your code where you want to start debugging. When the program runs and reaches this line, it pauses and opens an interactive prompt.

• import pdb: Loads the debugger module.
• pdb.set_trace(): Starts the debugger at this line.

python

import pdb

# Your code here
pdb.set_trace()  # Pause and open debugger
# More code

Example

This example shows how to use pdb.set_trace() to pause execution and inspect variables. When running, the program stops at the debugger prompt where you can type commands like n to go to the next line or p variable to print a variable's value.

python

import pdb

def add_numbers(a, b):
    pdb.set_trace()  # Start debugger here
    result = a + b
    return result

sum_value = add_numbers(3, 5)
print(f"Sum is {sum_value}")

Output

(Pdb) p a
3
(Pdb) p b
5
(Pdb) n
> <stdin>(6)add_numbers()
-> result = a + b
(Pdb) n
> <stdin>(7)add_numbers()
-> return result
(Pdb) n
Sum is 8

Common Pitfalls

Common mistakes when using pdb include:

• Forgetting to import pdb before calling set_trace().
• Leaving pdb.set_trace() in production code, which pauses the program unexpectedly.
• Not knowing debugger commands, which can make navigation confusing.

Always remove or comment out pdb.set_trace() after debugging.

python

import pdb

# Wrong: missing import
# pdb.set_trace()  # This will cause an error

# Right:
import pdb
pdb.set_trace()  # Correct usage

Quick Reference

Command	Description
n	Go to the next line of code
c	Continue execution until next breakpoint or program end
p variable	Print the value of a variable
l	List source code around the current line
q	Quit the debugger and stop the program

Key Takeaways

Import pdb and use pdb.set_trace() to start debugging at any code line.

Use debugger commands like n, p, c, l, and q to navigate and inspect your code.

Remove pdb.set_trace() calls before deploying your code to avoid unwanted pauses.

The pdb debugger helps find bugs by letting you pause and check variables step-by-step.