Python Debugging — Finding & Fixing Bugs

Debugging is the art of finding and fixing bugs. Python provides excellent tools for systematic debugging.

Learning Objectives

• Use pdb and breakpoint() for interactive debugging
• Apply logging for production debugging
• Use assertions for defensive programming
• Follow systematic debugging workflows

breakpoint() — Built-in Debugger

def calculate_discount(price, discount):
    breakpoint()  # Execution stops here
    discounted = price * (1 - discount)
    return discounted

# Commands in pdb:
# n (next)     — execute next line
# s (step)     — step into function
# c (continue) — run until next breakpoint
# p var        — print variable
# l (list)     — show source code
# q (quit)     — exit debugger

Logging for Debugging

import logging

logging.basicConfig(level=logging.DEBUG)

def process_order(order_id):
    logging.debug(f"Processing order {order_id}")

    order = get_order(order_id)
    logging.debug(f"Order details: {order}")

    total = calculate_total(order)
    logging.debug(f"Calculated total: {total}")

    return total

Assertions

def divide(a, b):
    assert b != 0, "Division by zero"
    return a / b

def process_list(items):
    assert isinstance(items, list), "Expected a list"
    assert len(items) > 0, "List cannot be empty"
    return sum(items) / len(items)

Systematic Debugging

# 1. Reproduce the bug
# 2. Read the error message carefully
# 3. Check the traceback
# 4. Print variable values
# 5. Simplify the code
# 6. Use debugger to step through
# 7. Check documentation
# 8. Search for similar issues
# 9. Fix and test
# 10. Add a test to prevent regression

Key Takeaways

1. Use breakpoint() for interactive debugging
2. Use logging instead of print for production code
3. Assertions catch bugs early in development
4. Read error messages — they usually tell you what is wrong
5. Simplify code to isolate the bug