Lambda vs regular functions in Python - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of lambda with default argument

What is the output of this code?

Python

funcs = [(lambda x=i: x*2) for i in range(3)]
results = [f() for f in funcs]
print(results)

A [0, 2, 4]

B [4, 4, 4]

C [0, 0, 0]

D TypeError

💡 Hint

Think about how default arguments capture values at definition time.

Each lambda captures the current value of i as a default argument, so calling each lambda returns double its own i value.

❓ Predict Output

intermediate

Difference in output between lambda and regular function

What is the output of this code?

Python

def make_funcs():
    funcs = []
    for i in range(3):
        def f():
            return i * 2
        funcs.append(f)
    return funcs

funcs = make_funcs()
results = [f() for f in funcs]
print(results)

A [0, 2, 4]

B [4, 4, 4]

C [0, 0, 0]

D NameError

💡 Hint

Consider when the variable i is looked up inside the function.

The inner function f uses the variable i from the outer scope, which after the loop ends is 2. So all functions return 2*2=4.

🔧 Debug

advanced

Identify the error in lambda with multiple statements

Which option shows the correct way to write a lambda that returns the sum of two numbers and prints a message?

A lambda x, y: (print('Adding'), x + y)[1]

B lambda x, y: print('Adding') + (x + y)

C lambda x, y: {print('Adding'); return x + y}

D lambda x, y: x + y; print('Adding')

💡 Hint

Remember lambdas can only have expressions, not statements.

Option A uses a tuple expression to execute print and then return the sum. Other options have syntax errors or invalid syntax for lambdas.

❓ Predict Output

advanced

Output of lambda capturing loop variable without default

What is the output of this code?

Python

funcs = []
for i in range(3):
    funcs.append(lambda: i)
results = [f() for f in funcs]
print(results)

A [0, 1, 2]

B [0, 0, 0]

C [2, 2, 2]

D SyntaxError

💡 Hint

Think about when the variable i is evaluated inside the lambda.

All lambdas refer to the same variable i, which after the loop ends is 2, so all return 2.

🧠 Conceptual

expert

Why use lambda instead of regular function?

Which of these is the best reason to use a lambda function instead of a regular function?

A To create functions that can be called without parentheses

B To improve performance by compiling faster than regular functions

C To allow multiple statements inside the function body

D To define a small anonymous function inline, especially as an argument to another function

💡 Hint

Think about where lambdas are often used in Python code.

Lambdas are used for small, unnamed functions, often passed as arguments. They cannot contain multiple statements and do not improve performance.

Practice

1. Which of the following best describes a lambda function in Python?

easy

A. A function defined using def that can have multiple statements.

B. A short, anonymous function defined with the lambda keyword.

C. A function that must always return None.

D. A function that can only be used inside classes.

Solution

1. Step 1: Understand lambda function definition

   A lambda function is defined using the lambda keyword and is anonymous (no name).

2. Step 2: Compare with regular functions

   Regular functions use def and can have multiple statements, unlike lambdas which are single expressions.

3. Final Answer:

   A short, anonymous function defined with the lambda keyword. -> Option B

4. Quick Check:

   Lambda = short anonymous function [OK]

Hint: Lambdas are short and nameless functions [OK]

Common Mistakes:

• Thinking lambdas can have multiple statements
• Confusing lambda with regular def functions
• Believing lambdas must return None

2. Which of the following is the correct syntax for a lambda function that adds 5 to its input?

easy

A. def add_five(x): return x + 5

B. lambda x x + 5

C. lambda x: x + 5

D. lambda (x): return x + 5

Solution

1. Step 1: Recall lambda syntax

   A lambda function is written as lambda parameters: expression without def or return.

2. Step 2: Check each option

   lambda x: x + 5 matches the correct syntax: lambda x: x + 5. Others have syntax errors or use def.

3. Final Answer:

   lambda x: x + 5 -> Option C

4. Quick Check:

   Lambda syntax = lambda params: expression [OK]

Hint: Lambda uses colon, no def or return [OK]

Common Mistakes:

• Including 'def' or 'return' in lambda
• Missing colon after parameters
• Using parentheses incorrectly

3. What is the output of the following code?

add = lambda x, y: x + y
def add_func(x, y):
    return x + y

print(add(3, 4))
print(add_func(3, 4))

medium

A. 7\n7

B. 34\n34

C. TypeError\nTypeError

D. None\nNone

Solution

1. Step 1: Understand lambda and regular function behavior

   Both add (lambda) and add_func (regular) add two numbers and return the sum.

2. Step 2: Evaluate the print statements

   Calling add(3, 4) and add_func(3, 4) both return 7, so output is two lines with 7.

3. Final Answer:

   7 7 -> Option A

4. Quick Check:

   Both add functions return sum = 7 [OK]

Hint: Both lambda and def return same result if logic matches [OK]

Common Mistakes:

• Thinking lambda returns string concatenation
• Confusing output with error messages
• Assuming lambda can't return values

4. Identify the error in this code snippet:

multiply = lambda x, y:
    x * y

print(multiply(2, 3))

medium

A. No error; output is 6

B. SyntaxError due to missing colon after lambda parameters

C. TypeError because lambda cannot take two arguments

D. IndentationError because lambda body is on new line

Solution

1. Step 1: Check lambda syntax rules

   Lambda functions must have their expression on the same line as the lambda keyword and parameters.

2. Step 2: Analyze the code structure

   The code places the expression x * y on the next line, causing an IndentationError.

3. Final Answer:

   IndentationError because lambda body is on new line -> Option D

4. Quick Check:

   Lambda body must be on same line [OK]

Hint: Lambda body must be on same line as parameters [OK]

Common Mistakes:

• Placing lambda body on next line
• Adding colon after lambda parameters
• Thinking lambda can't take multiple arguments

5. You want to sort a list of tuples by the second item using a function. Which is the best way to write the key function?

hard

A. list.sort(key=lambda t: t[1])

B. def get_second(t): return t[1]
list.sort(key=get_second(t))

C. list.sort(key=lambda t t[1])

D. list.sort(key=get_second())

Solution

1. Step 1: Understand sorting with key functions

   The key parameter expects a function that takes one argument and returns the value to sort by.

2. Step 2: Evaluate options for correctness and efficiency

   def get_second(t): return t[1] list.sort(key=get_second(t)) defines a regular function but incorrectly calls get_second(t) (causing NameError: 't' not defined). list.sort(key=lambda t: t[1]) uses a lambda inline, which is concise and common. list.sort(key=lambda t t[1]) has syntax error (missing colon). list.sort(key=get_second()) calls get_second() instead of passing the function.

3. Step 3: Choose best practice

   Using a lambda inline is best for simple, one-time use functions.

4. Final Answer:

   list.sort(key=lambda t: t[1]) -> Option A

5. Quick Check:

   Lambda inline for simple key function [OK]

Hint: Use lambda inline for simple sort keys [OK]

Common Mistakes:

• Forgetting colon in lambda
• Calling function instead of passing it
• Using multi-line def when lambda suffices