How Python executes code - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding Python Execution Order

What is the output of this Python code snippet?

Python

def greet():
    print("Hello")

print("Start")
greet()
print("End")

A Hello\nStart\nEnd

B Start\nHello\nEnd

C Start\nEnd\nHello

D Hello\nEnd\nStart

💡 Hint

Think about the order in which functions are called and print statements run.

Python runs code line by line. First it prints "Start", then calls greet() which prints "Hello", then prints "End".

❓ Predict Output

intermediate

Variable Scope in Python Execution

What will be the output of this code?

Python

x = 5
def change():
    x = 10
change()
print(x)

A 5

B 10

C None

D Error

💡 Hint

Consider where the variable x is changed and where it is printed.

The change() function creates a new local variable x. The global x remains 5, so print(x) outputs 5.

❓ Predict Output

advanced

Order of Execution with List Comprehensions

What is the output of this code?

Python

result = [x**2 for x in range(3)]
print(result)

A Error

B [1, 4, 9]

C [0, 1, 2]

D [0, 1, 4]

💡 Hint

Remember that range(3) produces 0,1,2 and each is squared.

The list comprehension squares each number from 0 to 2, resulting in [0, 1, 4].

❓ Predict Output

advanced

Execution Flow with Conditional Statements

What will this code print?

Python

x = 7
if x > 5:
    print("Greater")
else:
    print("Smaller")

A Error

B Smaller

C Greater

D Nothing

💡 Hint

Check the condition in the if statement carefully.

Since 7 is greater than 5, the if block runs and prints "Greater".

🧠 Conceptual

expert

Python Execution Model: What Happens First?

When Python runs a script, which step happens first?

A Python compiles the code to bytecode before running it

B Python immediately runs the code line by line without compilation

C Python converts the code to machine code directly

D Python sends the code to the operating system to execute

💡 Hint

Think about how Python prepares code before executing it.

Python first compiles the source code into bytecode, which is a lower-level, platform-independent representation. Then it runs this bytecode on the Python Virtual Machine.

Practice

1. What is the first step Python takes when you run a program?

easy

A. It converts the code into bytecode.

B. It immediately executes the code line by line.

C. It compiles the code into machine language.

D. It sends the code to the operating system.

Solution

1. Step 1: Understand Python's execution process

   Python first translates the source code into bytecode, which is a lower-level, platform-independent representation.

2. Step 2: Recognize what happens next

   The bytecode is then executed by the Python Virtual Machine (PVM), not the operating system directly.

3. Final Answer:

   It converts the code into bytecode. -> Option A

4. Quick Check:

   Python compiles to bytecode first [OK]

Hint: Remember: Python compiles before running [OK]

Common Mistakes:

• Thinking Python runs code directly without compiling
• Confusing bytecode with machine code
• Assuming OS runs Python code directly

2. Which of the following is the correct way to write a simple Python print statement?

easy

A. print "Hello, world!"

B. print('Hello, world!')

C. echo('Hello, world!')

D. printf('Hello, world!')

Solution

1. Step 1: Recall Python syntax for printing

   Python uses the print() function with parentheses and quotes around the string.

2. Step 2: Identify the correct syntax

   print('Hello, world!') uses print() with parentheses and quotes correctly.

3. Final Answer:

   print('Hello, world!') -> Option B

4. Quick Check:

   print() needs parentheses [OK]

Hint: Use print() with parentheses and quotes [OK]

Common Mistakes:

• Omitting parentheses in print statement
• Using echo or printf which are not Python functions
• Using print without quotes around text

3. What will be the output of this code?

print('Start')
for i in range(2):
    print(i)
print('End')

medium

A. Start\n0\n1\nEnd

B. Start\n1\n2\nEnd

C. 0\n1\nStart\nEnd

D. Start\nEnd

Solution

1. Step 1: Trace the print and loop execution

   The first print outputs 'Start'. The loop runs with i=0 and i=1, printing each. Then 'End' is printed.

2. Step 2: Write the output line by line

   Output lines are: Start, 0, 1, End.

3. Final Answer:

   Start\n0\n1\nEnd -> Option A

4. Quick Check:

   Loop prints 0 and 1 between Start and End [OK]

Hint: Remember range(2) gives 0 and 1 [OK]

Common Mistakes:

• Assuming range(2) starts at 1
• Ignoring the initial and final print statements
• Confusing loop variable values

4. Find the error in this code snippet:

for i in range(3)
    print(i)

medium

A. print() should be outside the loop

B. Indentation error on print statement

C. Missing colon ':' after range(3)

D. range() function is used incorrectly

Solution

1. Step 1: Check syntax of for loop

   Python requires a colon ':' at the end of the for statement line.

2. Step 2: Identify the missing colon

   The code misses ':' after range(3), causing a syntax error.

3. Final Answer:

   Missing colon ':' after range(3) -> Option C

4. Quick Check:

   for loops need ':' at the end [OK]

Hint: Look for missing colons in loops and conditionals [OK]

Common Mistakes:

• Forgetting the colon ':' after for statement
• Misplacing indentation but here it's correct
• Thinking range() usage is wrong

5. You want to write a program that prints numbers 1 to 5, but skips number 3. Which code correctly shows how Python executes this?

hard

A. for i in range(1,6):
if i == 3:
pass
print(i)

B. for i in range(1,5):
if i == 3:
break
print(i)

C. for i in range(1,6):
if i != 3:
break
print(i)

D. for i in range(1,6):
if i == 3:
continue
print(i)

Solution

1. Step 1: Understand the goal and loop behavior

   We want to print numbers 1 to 5 but skip printing 3. Using continue skips the current loop iteration.

2. Step 2: Analyze each option

   for i in range(1,6): if i == 3: continue print(i) uses continue to skip printing 3 and prints all others. Others either break early or print 3.

3. Final Answer:

   for i in range(1,6): if i == 3: continue print(i) -> Option D

4. Quick Check:

   continue skips printing 3 [OK]

Hint: Use continue to skip unwanted loop steps [OK]

Common Mistakes:

• Using break which stops the loop entirely
• Using pass which does nothing to skip printing
• Incorrect range limits causing missing numbers