Python Notes

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Python Comments

How to write comments in Python — single-line, multi-line, docstrings, and best practices to make code readable and maintainable.

Comments are text that Python ignores — they are only for the programmer, not the machine. Just as you write notes in a notebook, you write comments in code so that it makes sense later.

2. Single-Line Comments (#)

The # (hash / pound) symbol creates a single-line comment.

# creates a single-line comment. Everything after # is ignored by Python.

python example

# This is a single-line comment
print("Hello, World!")   # This is an inline comment

# You can use comments to explain variables
age = 25         # age in years
salary = 50000   # monthly salary in INR
tax_rate = 0.10  # 10% income tax

Output

Hello, World!

Comment Position Styles

Example

# ─── Style 1: Above the code (most common) ─── # Calculate the total price with tax price = 100 tax = 18 total = price + tax # ─── Style 2: Inline (same line) ─── discount = 10 # discount in percentage # ─── Style 3: Section Headers ─── # ========================================== # SECTION: User Authentication # ========================================== username = "admin" password = "secret123"

Output

3. Multi-Line Comments

Python has no official multi-line comment syntax. There are two approaches:

Approach 1: Multiple # Lines (Recommended)

python example

# This is a multi-line comment
# written using multiple hash symbols.
# Each line starts with a # character.
# This is the PEP 8 recommended approach.

def calculate_bmi(weight, height):
    # Body Mass Index formula:
    # BMI = weight(kg) / height(m)^2
    # Normal range: 18.5 - 24.9
    bmi = weight / (height ** 2)
    return round(bmi, 2)

print(calculate_bmi(70, 1.75))

Output

22.86

Approach 2: Triple-Quoted String (Unofficial)

python example

"""
This is a triple-quoted string.
It spans multiple lines.
Python doesn't execute it, so it acts like a comment.
But officially, it's a string object, not a comment!
"""

x = 10
print(x)

Output

You can write multi-line comment-like text with triple quotes, but technically it's a string object. Using multiple # for real multi-line comments is better practice.

Comparison Table

Feature	`#` Comment	Triple-Quote String
Official comment	✅ Yes	❌ No (it's a string)
Memory usage	None	Creates string object
PEP 8 recommended	✅ Yes	❌ Not for comments
Used as docstring	❌ No	✅ Yes (first statement)
Syntax highlighting	✅ Grayed out	Depends on editor

4. Docstrings — Documentation Strings

A docstring is a special string literal placed as the first statement of a module, function, class, or method.

A Docstring is a special string written at the beginning of a function, class, or module — it serves as official documentation accessible via help() and __doc__.

Function Docstring

python example

def greet(name, language="en"):
    """
    Greet a person in the specified language.

    Parameters:
        name (str): The person's name
        language (str): Language code — 'en' for English, 'hi' for Hindi

    Returns:
        str: Greeting message

    Example:
        >>> greet("Rahul")
        'Hello, Rahul!'
        >>> greet("Rahul", "hi")
        'Namaste, Rahul!'
    """
    if language == "hi":
        return f"Namaste, {name}!"
    return f"Hello, {name}!"

# Access docstring
print(greet("Rahul"))
print(greet("Rahul", "hi"))
print("\n--- Docstring ---")
print(greet.__doc__)

Output

Hello, Rahul!
Namaste, Rahul!

--- Docstring ---

    Greet a person in the specified language.

    Parameters:
        name (str): The person's name
        language (str): Language code — 'en' for English, 'hi' for Hindi

    Returns:
        str: Greeting message

    Example:
        >>> greet("Rahul")
        'Hello, Rahul!'
        >>> greet("Rahul", "hi")
        'Namaste, Rahul!'

Class Docstring

Example

class Student: """ Represents a student in the school management system. Attributes: name (str): Student's full name roll_no (int): Unique roll number grade (str): Current grade/class Methods: get_info(): Returns formatted student information update_grade(new_grade): Updates the student's grade """ def __init__(self, name, roll_no, grade): """Initialize student with name, roll number, and grade.""" self.name = name self.roll_no = roll_no self.grade = grade def get_info(self): """Return formatted student information string.""" return f"[{self.roll_no}] {self.name} — Grade: {self.grade}" s = Student("Priya Sharma", 101, "10th") print(s.get_info()) print("\n--- Class Doc ---") print(Student.__doc__)

Output

[101] Priya Sharma — Grade: 10th

--- Class Doc ---

    Represents a student in the school management system.
    ...

Module Docstring

python example

"""
school_utils.py
===============
Utility functions for the school management system.

Author: WoHoTech
Version: 1.0.0
Date: 2026-06-12
"""

# Module-level constant
SCHOOL_NAME = "WoHo Academy"

def get_school_name():
    """Return the school name."""
    return SCHOOL_NAME

# Access module docstring
print(__doc__)

5. Using help() with Docstrings

Example

def calculate_compound_interest(principal, rate, time, n=1): """ Calculate compound interest. Formula: A = P(1 + r/n)^(nt) Args: principal (float): Initial investment amount rate (float): Annual interest rate (as decimal, e.g. 0.05 for 5%) time (int): Time in years n (int): Number of times interest compounds per year (default: 1) Returns: float: Final amount after compound interest Examples: >>> calculate_compound_interest(1000, 0.05, 2) 1102.5 """ amount = principal * (1 + rate / n) ** (n * time) return round(amount, 2) # Using help() help(calculate_compound_interest)

Output

Help on function calculate_compound_interest in module __main__:

calculate_compound_interest(principal, rate, time, n=1)
    Calculate compound interest.

    Formula: A = P(1 + r/n)^(nt)

    Args:
        principal (float): Initial investment amount
        ...

6. Special Comments

Shebang Line (Unix/Linux)

python example

#!/usr/bin/env python3
# This tells the OS to run this script with Python 3
print("Running with Python 3")

Encoding Declaration

python example

# -*- coding: utf-8 -*-
# This is needed for non-ASCII characters in Python 2
# Python 3 defaults to UTF-8, but good practice to include

name = "रहुल"    # Hindi text
print(name)

Output

रहुल

TODO / FIXME / NOTE Comments

python example

def process_payment(amount):
    # TODO: Add currency conversion support
    # FIXME: This doesn't handle negative amounts
    # NOTE: Tax rate hardcoded — should come from config
    # HACK: Temporary fix until payment API is updated
    # OPTIMIZE: This loop runs O(n²) — needs improvement

    tax = amount * 0.18   # 18% GST
    total = amount + tax
    return total

result = process_payment(1000)
print(f"Total: ₹{result}")

Output

Total: ₹1180.0

7. Commenting Out Code (Debugging)

python example

def calculate(x, y):
    result = x + y

    # Debug: print intermediate values
    # print(f"x = {x}, y = {y}")
    # print(f"Before multiplication: {result}")

    result = result * 2

    return result

print(calculate(5, 3))

Output

When you want to temporarily disable some code during debugging, add #. This is called "commenting out" — don't delete it, you might need it later!

8. Docstring Formats/Styles

Different teams use different docstring formats:

Google Style (Most Popular)

python example

def divide(a, b):
    """Divide two numbers.

    Args:
        a (float): Numerator
        b (float): Denominator

    Returns:
        float: Result of a divided by b

    Raises:
        ZeroDivisionError: If b is zero

    Example:
        >>> divide(10, 2)
        5.0
    """
    if b == 0:
        raise ZeroDivisionError("Cannot divide by zero!")
    return a / b

print(divide(10, 2))
print(divide(15, 3))

Output

5.0
5.0

NumPy Style

python example

def add_numbers(a, b):
    """
    Add two numbers together.

    Parameters
    ----------
    a : int or float
        First number
    b : int or float
        Second number

    Returns
    -------
    int or float
        Sum of a and b
    """
    return a + b

reStructuredText Style (Sphinx)

python example

def multiply(a, b):
    """
    Multiply two numbers.

    :param a: First number
    :type a: int or float
    :param b: Second number
    :type b: int or float
    :return: Product of a and b
    :rtype: int or float
    """
    return a * b

9. ASCII Diagram — Comment Types

│ # -*- coding	utf-8 -*- ◄── Encoding
│ # TODO	improve later ◄── Special
│ # FIXME	bug here Marker

10. Best Practices Summary

✅ DO These

Example

# ✅ Good: Explain WHY, not WHAT # Using Fibonacci because it models this population growth pattern def fibonacci(n): if n <= 1: return n return fibonacci(n-1) + fibonacci(n-2) # ✅ Good: Update comments when code changes # Password must be 8+ chars with at least one digit (updated: 2026) MIN_PASSWORD_LENGTH = 8 # ✅ Good: Comment complex algorithms # Binary search: O(log n) - divide search space in half each iteration def binary_search(arr, target): left, right = 0, len(arr) - 1 while left <= right: mid = (left + right) // 2 if arr[mid] == target: return mid elif arr[mid] < target: left = mid + 1 else: right = mid - 1 return -1 result = binary_search([1, 3, 5, 7, 9, 11], 7) print(f"Found at index: {result}")

Output

Found at index: 3

❌ AVOID These

python example

# ❌ Bad: Stating the obvious
x = x + 1  # add 1 to x — unnecessary!

# ❌ Bad: Outdated comment
# Calculate tax at 10%  ← comment says 10% but code does 18%!
tax = price * 0.18

# ❌ Bad: Commented-out code left forever
# old_function()
# another_old_call()
# yet_another()
# (clean up — use git for history instead)

Practice Exercises

Exercise 1 — Add Comments to Code

Add meaningful comments to this code:

Example

def process(lst): result = [] for i in range(len(lst)): if lst[i] % 2 == 0: result.append(lst[i] ** 2) return result numbers = [1, 2, 3, 4, 5, 6, 7, 8] print(process(numbers))

Exercise 2 — Write Docstrings

Write a function calculate_grade(marks) that:

Takes marks (0-100) as input
Returns grade: A(90+), B(75+), C(60+), D(45+), F(below 45)
Has a complete Google-style docstring with examples

Exercise 3 — Module Documentation

Create a simple Python file calculator.py with:

Module-level docstring
4 functions (add, subtract, multiply, divide)
Each function has a docstring
Proper inline comments where needed

Interview Questions

Q1. What is the difference between a comment and a docstring in Python? > A comment starts with # and is completely ignored by Python — no memory allocation. A docstring is a string literal (using triple quotes) placed as the first statement of a function/class/module. It IS stored in the __doc__ attribute and accessible via help().

Q2. How do you write multi-line comments in Python? > Python has no official multi-line comment syntax. You can use multiple # lines (PEP 8 recommended) or triple-quoted strings (unofficial but common). For documentation purposes, use proper docstrings with triple quotes.

Q3. What is a docstring and how do you access it? > A docstring is a string literal placed as the first statement in a module, function, class, or method. Access it via function.__doc__ attribute or help(function). It follows PEP 257 conventions.

Q4. What special comment markers do professional developers use? > Common markers: TODO: (tasks to complete), FIXME: (known bugs), NOTE: (important information), HACK: (temporary workaround), OPTIMIZE: (performance improvements needed), DEPRECATED: (should not be used).

Q5. Why shouldn't you comment obvious code? > Comments should explain WHY code does something, not WHAT it does. Over-commenting adds noise and must be maintained. Good code should be self-documenting through clear variable/function names. Comments become outdated and misleading if not updated with code changes.

⚠️ Common Mistakes

Mistake 1: Obvious Comments Likhna 🙄

python example

# ❌ Bad — this is already obvious from reading the code
x = 10  # assigned 10 to x
name = "Rahul"  # stored Rahul in name variable

# ✅ Good — explain WHY, not WHAT
x = 10  # max retry attempts before timeout
name = "Rahul"  # default admin user for testing

If a comment merely repeats what the code does, it's unnecessary. Write "why" in comments, not "what"!

Mistake 2: Not Updating Comments 🕸️

python example

# ❌ Bad — comment says 10% but code does 18%
# Apply 10% discount
discount = price * 0.18

# ✅ Good — comment matches the code
# Apply 18% GST tax
tax = price * 0.18

When you change code, update the comments too. Outdated comments are misleading — they can be more dangerous than bugs!

Mistake 3: Commented Code Permanently Chhod Dena 🗑️

python example

# ❌ Bad — purana code permanently commented
# result = old_calculation(x, y)
# temp = result * 2
# print(temp)
result = new_calculation(x, y)

# ✅ Good — use Git for history, keep code clean
result = new_calculation(x, y)

Use Git version control! Commented-out code makes the codebase dirty. If you need it in the future, you can retrieve it from git history.

Mistake 4: Triple Quotes Ko Comment Samajhna 🤔

python example

# ❌ Bad — this is not a comment, it's a string object (uses memory)
"""
This is not really a comment.
Python creates a string object for this.
"""

# ✅ Good — real multi-line comment
# This is a proper multi-line comment
# Python completely ignores these lines
# No memory allocation happens

Triple-quoted strings are actual string objects stored in memory. Real comments are only created with #. Except for docstrings, don't use triple quotes as comments — it wastes memory.

Mistake 5: Incorrect Placement of Docstrings 📍

python example

# ❌ Bad — docstring must be FIRST statement
def greet(name):
    x = 10
    """This won't be accessible as __doc__"""
    return f"Hello, {name}"

# ✅ Good — docstring as first statement
def greet(name):
    """Greet a person by name and return the message."""
    x = 10
    return f"Hello, {name}"

A docstring must always be the first statement inside a function/class. If written elsewhere, __doc__ and help() won't work!

Mistake 6: Inline Comments Mein No Space 🔤

python example

# ❌ Bad — hard to read, PEP 8 violation
x = 10#this is bad
y = 20  #this is also bad

# ✅ Good — 2 spaces before #, 1 space after #
x = 10  # max connections allowed
y = 20  # timeout in seconds

PEP 8 says: put at least 2 spaces before an inline comment, and 1 space after #. Readability matters!

Mistake 7: Comments Mein Secret Information 🔒

python example

# ❌ NEVER DO THIS — passwords/keys in comments
# API Key: sk-abc123xyz (for production)
# DB Password: admin@123

# ✅ Good — use environment variables
# API key loaded from .env file (see .env.example for format)
import os
api_key = os.getenv("API_KEY")

Never write passwords, API keys, or sensitive data in comments! These can be publicly visible in code repositories. Use environment variables or secret managers instead.

✅ Key Takeaways

📝 Comments are created with # — Python completely ignores them, no memory is used.
📖 Docstrings are created with triple quotes ("""...""") — written as the first statement and accessible via __doc__ / help().
🎯 Explain WHY, not WHAT — write the reason for the logic in comments, not a translation of the code.
🔄 Update comments along with code — outdated comments are more dangerous than bugs.
📋 Follow PEP 8 — 2 spaces before inline comments, 1 space after #.
📚 Follow PEP 257 for docstrings — choose Google Style, NumPy Style, or reStructuredText Style and be consistent.
🏷️ Use special markers — TODO:, FIXME:, NOTE:, HACK:, OPTIMIZE: are helpful for team collaboration.
🚫 Don't leave commented-out code permanently — use Git for history, keep the codebase clean.
🔐 Never write sensitive data in comments — passwords and API keys should be in environment variables.
⚡ Write self-documenting code — good variable/function names reduce the need for comments.

❓ FAQ

Q1. Is there any official multi-line comment syntax in Python? > ❌ No! Python officially only has single-line comments (#). Writing multi-line comments using multiple # lines is the PEP 8 recommended approach. Triple-quoted strings are technically string objects, not comments.

Q2. What is the fundamental difference between a docstring and a comment? > A comment (#) is completely ignored by Python — it's neither executed nor stored in memory. A Docstring is an actual string object stored in the __doc__ attribute — accessible at runtime via help() and __doc__.

Q3. Do comments affect code performance/speed? > No! # comments have zero runtime impact because the Python interpreter skips them during the compilation phase. There's no trace of comments in bytecode. So don't worry about adding comments affecting performance.

Q4. Google Style, NumPy Style, reStructuredText — kaunsa docstring format use karna chahiye? > It depends on the team/project. Google Style is the most popular and readable — best for beginners. NumPy Style is common in data science projects. reStructuredText is used with Sphinx documentation. Pick one and be consistent.

Q5. Kya ek file mein sab jagah comments likhne chahiye? > No! Over-commenting is also a problem. Write comments only where the code's intent (purpose) isn't clear. Write self-documenting code — good variable names (total_price not tp) reduce the need for comments.

Q6. What is the role of comments like # type: ignore and # noqa? > These are special tool-specific comments. # type: ignore tells mypy (type checker) to skip this line. # noqa tells flake8/linter to suppress warnings on this line. They're not regular comments — they're directives for specific tools.

Q7. What is the benefit of writing examples in docstrings? > Huge benefit! Examples written with >>> in docstrings can be automatically tested by the doctest module: python -m doctest your_file.py. This becomes living documentation that also serves as tests.

Q8. Can you use Hindi or regional language in comments? > Technically yes — Python 3 supports UTF-8 so there won't be errors. But in professional projects, writing comments in English is standard because international teams need to understand the code.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Python Comments.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Python Master Course topic.

Search Terms

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Continue learning this concept

BasicsPython Data TypesPython ke sabhi built-in data types — int, float, str, bool, list, tuple, set, dict, None — ko deeply samjhein examples, diagrams aur comparisons ke saath.BasicsPython Input and OutputPython mein user se input lena aur output dikhana — input(), print(), f-strings, formatting, file I/O basics aur real-world input handling ke saath complete guide.BasicsPython OperatorsPython ke sabhi operators — arithmetic, comparison, logical, bitwise, assignment, identity, membership, aur walrus — examples aur precedence table ke saath complete guide.BasicsPython SyntaxPython syntax ke rules aur structure ko samjhein — indentation, statements, keywords, identifiers aur code likhne ka sahi tarika.BasicsPython Strings BasicsPython strings ko deeply samjhein — creation, indexing, slicing, built-in methods, formatting, escape sequences aur real-world string manipulation ke saath complete beginners guide.BasicsPython Type CastingPython mein ek data type se doosre mein convert karna — int(), float(), str(), bool(), list(), tuple(), set(), dict() — implicit aur explicit casting ke saath complete guide.

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Python Notes

Python Comments

How to write comments in Python — single-line, multi-line, docstrings, and best practices to make code readable and maintainable.

Comments are text that Python ignores — they are only for the programmer, not the machine. Just as you write notes in a notebook, you write comments in code so that it makes sense later.

2. Single-Line Comments (#)

The # (hash / pound) symbol creates a single-line comment.

# creates a single-line comment. Everything after # is ignored by Python.

python example

# This is a single-line comment
print("Hello, World!")   # This is an inline comment

# You can use comments to explain variables
age = 25         # age in years
salary = 50000   # monthly salary in INR
tax_rate = 0.10  # 10% income tax

Output

Hello, World!

Comment Position Styles

Example

Output

3. Multi-Line Comments

Python has no official multi-line comment syntax. There are two approaches:

Approach 1: Multiple # Lines (Recommended)

python example

# This is a multi-line comment
# written using multiple hash symbols.
# Each line starts with a # character.
# This is the PEP 8 recommended approach.

def calculate_bmi(weight, height):
    # Body Mass Index formula:
    # BMI = weight(kg) / height(m)^2
    # Normal range: 18.5 - 24.9
    bmi = weight / (height ** 2)
    return round(bmi, 2)

print(calculate_bmi(70, 1.75))

Output

22.86

Approach 2: Triple-Quoted String (Unofficial)

python example

"""
This is a triple-quoted string.
It spans multiple lines.
Python doesn't execute it, so it acts like a comment.
But officially, it's a string object, not a comment!
"""

x = 10
print(x)

Output

You can write multi-line comment-like text with triple quotes, but technically it's a string object. Using multiple # for real multi-line comments is better practice.

Comparison Table

Feature	`#` Comment	Triple-Quote String
Official comment	✅ Yes	❌ No (it's a string)
Memory usage	None	Creates string object
PEP 8 recommended	✅ Yes	❌ Not for comments
Used as docstring	❌ No	✅ Yes (first statement)
Syntax highlighting	✅ Grayed out	Depends on editor

4. Docstrings — Documentation Strings

A docstring is a special string literal placed as the first statement of a module, function, class, or method.

A Docstring is a special string written at the beginning of a function, class, or module — it serves as official documentation accessible via help() and __doc__.

Function Docstring

python example

def greet(name, language="en"):
    """
    Greet a person in the specified language.

    Parameters:
        name (str): The person's name
        language (str): Language code — 'en' for English, 'hi' for Hindi

    Returns:
        str: Greeting message

    Example:
        >>> greet("Rahul")
        'Hello, Rahul!'
        >>> greet("Rahul", "hi")
        'Namaste, Rahul!'
    """
    if language == "hi":
        return f"Namaste, {name}!"
    return f"Hello, {name}!"

# Access docstring
print(greet("Rahul"))
print(greet("Rahul", "hi"))
print("\n--- Docstring ---")
print(greet.__doc__)

Output

Hello, Rahul!
Namaste, Rahul!

--- Docstring ---

    Greet a person in the specified language.

    Parameters:
        name (str): The person's name
        language (str): Language code — 'en' for English, 'hi' for Hindi

    Returns:
        str: Greeting message

    Example:
        >>> greet("Rahul")
        'Hello, Rahul!'
        >>> greet("Rahul", "hi")
        'Namaste, Rahul!'

Class Docstring

Example

Output

[101] Priya Sharma — Grade: 10th

--- Class Doc ---

    Represents a student in the school management system.
    ...

Module Docstring

python example

"""
school_utils.py
===============
Utility functions for the school management system.

Author: WoHoTech
Version: 1.0.0
Date: 2026-06-12
"""

# Module-level constant
SCHOOL_NAME = "WoHo Academy"

def get_school_name():
    """Return the school name."""
    return SCHOOL_NAME

# Access module docstring
print(__doc__)

5. Using help() with Docstrings

Example

Output

Help on function calculate_compound_interest in module __main__:

calculate_compound_interest(principal, rate, time, n=1)
    Calculate compound interest.

    Formula: A = P(1 + r/n)^(nt)

    Args:
        principal (float): Initial investment amount
        ...

6. Special Comments

Shebang Line (Unix/Linux)

python example

#!/usr/bin/env python3
# This tells the OS to run this script with Python 3
print("Running with Python 3")

Encoding Declaration

python example

# -*- coding: utf-8 -*-
# This is needed for non-ASCII characters in Python 2
# Python 3 defaults to UTF-8, but good practice to include

name = "रहुल"    # Hindi text
print(name)

Output

रहुल

TODO / FIXME / NOTE Comments

python example

def process_payment(amount):
    # TODO: Add currency conversion support
    # FIXME: This doesn't handle negative amounts
    # NOTE: Tax rate hardcoded — should come from config
    # HACK: Temporary fix until payment API is updated
    # OPTIMIZE: This loop runs O(n²) — needs improvement

    tax = amount * 0.18   # 18% GST
    total = amount + tax
    return total

result = process_payment(1000)
print(f"Total: ₹{result}")

Output

Total: ₹1180.0

7. Commenting Out Code (Debugging)

python example

def calculate(x, y):
    result = x + y

    # Debug: print intermediate values
    # print(f"x = {x}, y = {y}")
    # print(f"Before multiplication: {result}")

    result = result * 2

    return result

print(calculate(5, 3))

Output

When you want to temporarily disable some code during debugging, add #. This is called "commenting out" — don't delete it, you might need it later!

8. Docstring Formats/Styles

Different teams use different docstring formats:

Google Style (Most Popular)

python example

def divide(a, b):
    """Divide two numbers.

    Args:
        a (float): Numerator
        b (float): Denominator

    Returns:
        float: Result of a divided by b

    Raises:
        ZeroDivisionError: If b is zero

    Example:
        >>> divide(10, 2)
        5.0
    """
    if b == 0:
        raise ZeroDivisionError("Cannot divide by zero!")
    return a / b

print(divide(10, 2))
print(divide(15, 3))

Output

5.0
5.0

NumPy Style

python example

def add_numbers(a, b):
    """
    Add two numbers together.

    Parameters
    ----------
    a : int or float
        First number
    b : int or float
        Second number

    Returns
    -------
    int or float
        Sum of a and b
    """
    return a + b

reStructuredText Style (Sphinx)

python example

def multiply(a, b):
    """
    Multiply two numbers.

    :param a: First number
    :type a: int or float
    :param b: Second number
    :type b: int or float
    :return: Product of a and b
    :rtype: int or float
    """
    return a * b

9. ASCII Diagram — Comment Types

│ # -*- coding	utf-8 -*- ◄── Encoding
│ # TODO	improve later ◄── Special
│ # FIXME	bug here Marker

10. Best Practices Summary

✅ DO These

Example

Output

Found at index: 3

❌ AVOID These

python example

# ❌ Bad: Stating the obvious
x = x + 1  # add 1 to x — unnecessary!

# ❌ Bad: Outdated comment
# Calculate tax at 10%  ← comment says 10% but code does 18%!
tax = price * 0.18

# ❌ Bad: Commented-out code left forever
# old_function()
# another_old_call()
# yet_another()
# (clean up — use git for history instead)

Practice Exercises

Exercise 1 — Add Comments to Code

Add meaningful comments to this code:

Example

def process(lst): result = [] for i in range(len(lst)): if lst[i] % 2 == 0: result.append(lst[i] ** 2) return result numbers = [1, 2, 3, 4, 5, 6, 7, 8] print(process(numbers))

Exercise 2 — Write Docstrings

Write a function calculate_grade(marks) that:

Takes marks (0-100) as input
Returns grade: A(90+), B(75+), C(60+), D(45+), F(below 45)
Has a complete Google-style docstring with examples

Exercise 3 — Module Documentation

Create a simple Python file calculator.py with:

Module-level docstring
4 functions (add, subtract, multiply, divide)
Each function has a docstring
Proper inline comments where needed

Interview Questions

⚠️ Common Mistakes

Mistake 1: Obvious Comments Likhna 🙄

python example

# ❌ Bad — this is already obvious from reading the code
x = 10  # assigned 10 to x
name = "Rahul"  # stored Rahul in name variable

# ✅ Good — explain WHY, not WHAT
x = 10  # max retry attempts before timeout
name = "Rahul"  # default admin user for testing

If a comment merely repeats what the code does, it's unnecessary. Write "why" in comments, not "what"!

Mistake 2: Not Updating Comments 🕸️

python example

# ❌ Bad — comment says 10% but code does 18%
# Apply 10% discount
discount = price * 0.18

# ✅ Good — comment matches the code
# Apply 18% GST tax
tax = price * 0.18

When you change code, update the comments too. Outdated comments are misleading — they can be more dangerous than bugs!

Mistake 3: Commented Code Permanently Chhod Dena 🗑️

python example

# ❌ Bad — purana code permanently commented
# result = old_calculation(x, y)
# temp = result * 2
# print(temp)
result = new_calculation(x, y)

# ✅ Good — use Git for history, keep code clean
result = new_calculation(x, y)

Use Git version control! Commented-out code makes the codebase dirty. If you need it in the future, you can retrieve it from git history.

Mistake 4: Triple Quotes Ko Comment Samajhna 🤔

python example

# ❌ Bad — this is not a comment, it's a string object (uses memory)
"""
This is not really a comment.
Python creates a string object for this.
"""

# ✅ Good — real multi-line comment
# This is a proper multi-line comment
# Python completely ignores these lines
# No memory allocation happens

Triple-quoted strings are actual string objects stored in memory. Real comments are only created with #. Except for docstrings, don't use triple quotes as comments — it wastes memory.

Mistake 5: Incorrect Placement of Docstrings 📍

python example

# ❌ Bad — docstring must be FIRST statement
def greet(name):
    x = 10
    """This won't be accessible as __doc__"""
    return f"Hello, {name}"

# ✅ Good — docstring as first statement
def greet(name):
    """Greet a person by name and return the message."""
    x = 10
    return f"Hello, {name}"

A docstring must always be the first statement inside a function/class. If written elsewhere, __doc__ and help() won't work!

Mistake 6: Inline Comments Mein No Space 🔤

python example

# ❌ Bad — hard to read, PEP 8 violation
x = 10#this is bad
y = 20  #this is also bad

# ✅ Good — 2 spaces before #, 1 space after #
x = 10  # max connections allowed
y = 20  # timeout in seconds

PEP 8 says: put at least 2 spaces before an inline comment, and 1 space after #. Readability matters!

Mistake 7: Comments Mein Secret Information 🔒

python example

# ❌ NEVER DO THIS — passwords/keys in comments
# API Key: sk-abc123xyz (for production)
# DB Password: admin@123

# ✅ Good — use environment variables
# API key loaded from .env file (see .env.example for format)
import os
api_key = os.getenv("API_KEY")

Never write passwords, API keys, or sensitive data in comments! These can be publicly visible in code repositories. Use environment variables or secret managers instead.

✅ Key Takeaways

📝 Comments are created with # — Python completely ignores them, no memory is used.
📖 Docstrings are created with triple quotes ("""...""") — written as the first statement and accessible via __doc__ / help().
🎯 Explain WHY, not WHAT — write the reason for the logic in comments, not a translation of the code.
🔄 Update comments along with code — outdated comments are more dangerous than bugs.
📋 Follow PEP 8 — 2 spaces before inline comments, 1 space after #.
📚 Follow PEP 257 for docstrings — choose Google Style, NumPy Style, or reStructuredText Style and be consistent.
🏷️ Use special markers — TODO:, FIXME:, NOTE:, HACK:, OPTIMIZE: are helpful for team collaboration.
🚫 Don't leave commented-out code permanently — use Git for history, keep the codebase clean.
🔐 Never write sensitive data in comments — passwords and API keys should be in environment variables.
⚡ Write self-documenting code — good variable/function names reduce the need for comments.

❓ FAQ

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Python Comments.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Python Master Course topic.

Search Terms

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