Comments in Python - Step-by-Step Execution

Concept Flow - Comments in Python

Start of code

↓

Read line

↓

Is line a comment?

Yes→Ignore line, no action

No↓

Execute line

↓

More lines?

Yes→Read next line

No↓

End of code

Python reads each line. If it sees a comment (starting with #), it skips it and does not run it. Otherwise, it runs the line.

Execution Sample

Python

# This is a comment
print('Hello')  # This prints Hello
# Another comment

This code shows comments ignored and a print statement executed.

Execution Table

Step	Line Read	Is Comment?	Action	Output
1	# This is a comment	Yes	Ignore line
2	print('Hello') # This prints Hello	No	Execute print	Hello
3	# Another comment	Yes	Ignore line

💡 All lines processed; comments ignored, print executed once.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Output	""	""	"Hello"	"Hello"	"Hello"

Key Moments - 2 Insights

Why doesn't the comment line produce any output?

Because the execution_table shows that comment lines are detected and ignored, so no action or output happens for them.

Does a comment after code on the same line affect the code?

No, as seen in step 2, the code before the # runs normally, and the comment after # is ignored.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 1?

A The comment line is ignored

B The comment line is executed

C The print function runs

D An error occurs

💡 Hint

Check the 'Is Comment?' and 'Action' columns for step 1 in the execution_table.

At which step does the program print 'Hello'?

A Step 1

B Step 3

C Step 2

D No step prints 'Hello'

💡 Hint

Look at the 'Output' column in the execution_table for each step.

If we remove the # from the first line, what changes in the execution_table?

A Step 1 will still be ignored

B Step 1 will execute code instead of ignoring

C Step 2 will not run

D No change at all

💡 Hint

Removing # means the line is no longer a comment, so check the 'Is Comment?' and 'Action' columns.

Concept Snapshot

Comments in Python start with # and are ignored by the computer.
They can be on their own line or after code.
Comments help explain code but do not run.
Use # to add notes without affecting program output.

Full Transcript

Python reads code line by line. When it sees a line starting with #, it treats it as a comment and skips running it. Comments can also appear after code on the same line; everything after # is ignored. Only lines without # at the start are executed. In the example, two comment lines are ignored, and one print statement runs, showing 'Hello'. This helps programmers add notes without changing how the program works.

Practice

1. What symbol is used to write a single-line comment in Python?

easy

A. /* */

B. //

C. #

D.

Solution

1. Step 1: Identify the comment symbol in Python

   Python uses the # symbol to start a single-line comment.

2. Step 2: Compare with other languages

   Other symbols like // or /* */ are used in languages like JavaScript or C, not Python.

3. Final Answer:

   # -> Option C

4. Quick Check:

   Single-line comment = # [OK]

Hint: Single-line comments start with # in Python [OK]

Common Mistakes:

• Using // instead of #
• Using /* */ for single-line comments
• Confusing HTML comments with Python

2. Which of the following is the correct way to write a multi-line comment in Python?

easy

A.

B. '''This is a multi-line comment'''

C. // This is a multi-line comment //

D. /* This is a multi-line comment */

Solution

1. Step 1: Recognize Python multi-line comment style

   Python uses triple single quotes ''' or triple double quotes """ for multi-line comments or docstrings.

2. Step 2: Eliminate other language comment styles

   Options A, B, and D are comment styles from other languages like HTML, JavaScript/C++, and C.

3. Final Answer:

   '''This is a multi-line comment''' -> Option B

4. Quick Check:

   Multi-line comment = triple quotes [OK]

Hint: Use triple quotes ''' or """ for multi-line comments [OK]

Common Mistakes:

• Using // or /* */ which are not Python syntax
• Using HTML style comments
• Trying to use # for multi-line comments without repeating

3. What will be the output of the following code?

print('Hello')  # This prints Hello
# print('World')

medium

A. Hello

B. SyntaxError

C. World

D. Hello\nWorld

Solution

1. Step 1: Understand the effect of comments on code execution

   The first line prints 'Hello' because the comment after it does not affect the print statement.

2. Step 2: Recognize that the second print is commented out

   The second print statement is fully commented out, so it does not run or print anything.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Commented code does not run = Hello [OK]

Hint: Code after # is ignored; only uncommented lines run [OK]

Common Mistakes:

• Thinking commented print runs
• Expecting both Hello and World printed
• Confusing comment with string

4. Find the error in this code snippet:

# This is a comment
print('Start')
'''This is a comment without an end
print('End')

medium

A. Comments cannot be placed before print statements

B. No error, code runs fine

C. print('End') is outside comment and runs normally

D. Missing closing triple quotes causes SyntaxError

Solution

1. Step 1: Check triple quote usage

   The triple quotes start a multi-line comment but are not closed, causing a syntax error.

2. Step 2: Understand impact on code parsing

   Without closing quotes, Python treats the rest as part of the string, leading to a SyntaxError.

3. Final Answer:

   Missing closing triple quotes causes SyntaxError -> Option D

4. Quick Check:

   Unclosed triple quotes = SyntaxError [OK]

Hint: Always close triple quotes to avoid syntax errors [OK]

Common Mistakes:

• Ignoring missing triple quote closure
• Thinking comments can be unclosed
• Assuming code after triple quotes runs

5. You want to temporarily disable a block of code without deleting it. Which is the best way to do this in Python?

hard

A. Put # at the start of each line in the block

B. Surround the code block with triple quotes ''' ... '''

C. Use /* ... */ around the block

D. Wrap the block in

Solution

1. Step 1: Understand Python comment styles for disabling code

   Python does not officially support block comments; the common way is to put # at the start of each line.

2. Step 2: Evaluate triple quotes usage

   Triple quotes create a string literal, which may not disable code properly and can cause unexpected behavior if used improperly.

3. Step 3: Recognize other language comment styles

   Options C and D are not valid in Python.

4. Final Answer:

   Put # at the start of each line in the block -> Option A

5. Quick Check:

   Disable code block = # per line [OK]

Hint: Use # on each line to disable code safely [OK]

Common Mistakes:

• Using triple quotes which may not disable code
• Using /* */ which is invalid in Python
• Using HTML comments which do nothing in Python