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What are comments?

Comments are notes for humans that Python ignores. They’re like sticky notes in your code that explain what’s happening. Why write comments?
  • Help others understand your code
  • Help future you remember what you did
  • Disable code temporarily
  • Document complex logic

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Single-line comments

Use # to start a comment: 
# This is a comment
print("Hello")  # This is also a comment

# You can have multiple lines
# of comments by starting
# each line with a hash

age = 25  # Store user's age

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Multi-line comments

For longer explanations, use triple quotes: 
"""
This is a multi-line comment.
It can span several lines.
Great for longer explanations.
"""

def calculate_tip(bill):
    """
    Calculate 20% tip for a restaurant bill.
    Takes the bill amount and returns the tip.
    """
    return bill * 0.20
Technically, triple quotes create a string that Python doesn’t use. Real multi-line comments don’t exist in Python, but this works the same way!

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When to use comments

Good comments explain WHY, not WHAT: 
# Good: Explains why
# Using 1.0625 because sales tax in CA is 6.25%
total = subtotal * 1.0625

# Bad: States the obvious
# Multiply subtotal by 1.0625
total = subtotal * 1.0625

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Best practices

Use descriptive variable names instead of comments when possible. days_in_week = 7 is clearer than d = 7 # days.

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Temporary comments

Use comments to disable code temporarily: 
print("Starting process...")
# print("Debug info")  # Uncomment for debugging
new_method()
# old_method()  # Keeping for reference

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Common mistakes

# Bad: Says 15% but does 20%
# Calculate 15% tip
tip = bill * 0.20

# Good: Accurate
# Calculate 20% tip (standard)
tip = bill * 0.20

# Bad: Too obvious
x = 5  # Set x to 5
y = 10  # Set y to 10

# Good: Only explain complex parts
x = 5
y = 10
position = x + y  # Convert to linear index

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What’s next?

Now that you know the basics and how to document your code, let’s explore the different types of data Python can work with!

Data types

Numbers, text, and more