Debugging#

In programming, bugs are errors that cause a program to behave incorrectly or crash. Debugging is the process of finding and fixing these bugs.

Types of Errors#

  1. Syntax Errors: Mistakes in code structure, detected before execution.

if x == 5
    print("Missing colon")

  1. Runtime Errors: Errors that occur during execution (e.g., division by zero).

result = 10 / 0  # ZeroDivisionError

  1. Logical Errors: The program runs but produces incorrect results due to flawed logic.

def average(a: int, b: int) -> float:
    return (a + b) / 3  # Incorrect formula

Using print for Debugging#

A simple and common way to debug is to print the values of variables:

def greet(name: str) -> str:
    print(f"Debug: name = {name}")  # Debug print
    return f"Hello, {name}!"

greet("Alice")

Using Debugger in IDEs#

Modern IDEs like VSCode and PyCharm provide powerful visual debuggers:

  • Breakpoints: Pause execution at a specific line.

  • Step Into: Move into a function call.

  • Step Over: Execute the current line and move to the next.

  • Step Out: Finish the current function and return to the caller.

  • Watch Window: Monitor specific variables.

  • Call Stack: View the sequence of function calls.

Using the pdb Module#

Python has a built-in debugger called pdb:

import pdb

def divide(a: int, b: int) -> float:
    pdb.set_trace()
    return a / b

divide(10, 2)

When run, it opens an interactive prompt:

  • n: next line

  • c: continue

  • q: quit

  • p variable: print variable’s value

Understanding Tracebacks#

When an exception occurs, Python provides a traceback — a detailed report of the call stack leading to the error.

def f1():
    return 10 / 0

def f2():
    f1()

f2()

The traceback will show the exact file, line number, and function call that caused the error.

Effective Debugging Techniques#

  • Reproduce the bug consistently.

  • Use print() or logging to inspect variables.

  • Minimize code to isolate the issue.

  • Use a debugger (pdb or IDE).

  • Read tracebacks carefully to locate the source.

  • Check assumptions (types, values, ranges).

Additional Resources#

Check the official Python documentation on pdb