Lambda syntax and behavior in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple lambda function

What is the output of this Python code?

Python

func = lambda x: x * 3
result = func(4)
print(result)

A 12

B 7

C Error

D None

💡 Hint

Remember that lambda functions return the expression result.

The lambda multiplies the input by 3. Input is 4, so 4 * 3 = 12.

❓ Predict Output

intermediate

Lambda with multiple arguments

What will this code print?

Python

adder = lambda a, b: a + b
print(adder(5, 10))

A 15

B 510

C TypeError

D None

💡 Hint

The lambda adds two numbers.

The lambda adds 5 and 10, resulting in 15.

❓ Predict Output

advanced

Lambda capturing variable in loop

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 [2, 2, 2]

C [0, 0, 0]

D TypeError

💡 Hint

Lambdas capture variables by reference, not by value.

All lambdas capture the same variable i, which ends at 2 after the loop.

❓ Predict Output

advanced

Correct lambda capturing loop variable

What does this code print?

Python

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

A [0, 0, 0]

B [2, 2, 2]

C [0, 1, 2]

D SyntaxError

💡 Hint

Default arguments capture the current value at each loop iteration.

Using default argument i=i captures the current value of i for each lambda.

🧠 Conceptual

expert

Behavior of lambda with mutable default argument

Consider this code snippet. What will be printed?

Python

def make_funcs():
    funcs = []
    lst = []
    for i in range(3):
        lst.append(i)
        funcs.append(lambda x=lst: x)
    return funcs

funcs = make_funcs()
for f in funcs:
    print(f())

A TypeError

B

[0]
[0, 1]
[0, 1, 2]

C

[]
[]
[]

D

[0, 1, 2]
[0, 1, 2]
[0, 1, 2]

💡 Hint

Default arguments are evaluated once when the function is defined.

The default argument x=lst captures the same list object, which has all elements at the end.

Practice

1. What does a lambda function in Python do?

easy

A. Creates a small anonymous function with a single expression

B. Defines a class with methods

C. Declares a variable

D. Imports a module

Solution

1. Step 1: Understand lambda purpose

   A lambda function is a quick way to create a function without a name that returns the result of one expression.

2. Step 2: Compare options

   Only Creates a small anonymous function with a single expression describes this behavior correctly. Other options describe unrelated Python features.

3. Final Answer:

   Creates a small anonymous function with a single expression -> Option A

4. Quick Check:

   Lambda = anonymous single-expression function [OK]

Hint: Lambdas are short unnamed functions with one expression [OK]

Common Mistakes:

• Thinking lambda can have multiple statements
• Confusing lambda with class or variable definitions
• Assuming lambda needs a name

2. Which of the following is the correct syntax for a lambda function that adds two numbers x and y?

easy

A. lambda (x, y) { return x + y }

B. lambda x, y: x + y

C. def lambda(x, y): return x + y

D. lambda x, y => x + y

Solution

1. Step 1: Recall lambda syntax

   In Python, lambda syntax is: lambda parameters: expression

2. Step 2: Check each option

   lambda x, y: x + y matches correct syntax. Options B and D use other language styles. def lambda(x, y): return x + y wrongly uses def with lambda.

3. Final Answer:

   lambda x, y: x + y -> Option B

4. Quick Check:

   Correct lambda syntax = lambda params: expression [OK]

Hint: Remember: lambda params colon expression [OK]

Common Mistakes:

• Using parentheses around parameters in lambda
• Trying to use curly braces or return keyword
• Mixing lambda with def syntax

3. What is the output of this code?

func = lambda x: x * 2
print(func(5))

medium

A. 10

B. 25

C. SyntaxError

D. None

Solution

1. Step 1: Understand the lambda function

   The lambda takes input x and returns x multiplied by 2.

2. Step 2: Calculate func(5)

   func(5) = 5 * 2 = 10

3. Final Answer:

   10 -> Option A

4. Quick Check:

   5 * 2 = 10 [OK]

Hint: Multiply input by 2 as lambda defines [OK]

Common Mistakes:

• Confusing multiplication with addition
• Expecting syntax error due to lambda
• Thinking lambda returns None by default

4. Find the error in this lambda function:

double = lambda x: return x * 2
print(double(4))

medium

A. No error, code runs fine

B. Missing parentheses around x

C. Lambda cannot multiply numbers

D. Using 'return' inside lambda causes SyntaxError

Solution

1. Step 1: Check lambda syntax rules

   Lambdas cannot contain statements like 'return'; they only have an expression.

2. Step 2: Identify error in code

   The code uses 'return' inside lambda, which is invalid and causes SyntaxError.

3. Final Answer:

   Using 'return' inside lambda causes SyntaxError -> Option D

4. Quick Check:

   Lambda disallows 'return' keyword [OK]

Hint: Lambdas never use 'return' keyword [OK]

Common Mistakes:

• Adding 'return' inside lambda
• Thinking parentheses are mandatory around parameters
• Believing lambda can't do math operations

5. You want to create a list of squares for numbers 1 to 5 using a lambda inside map. Which code correctly does this?

hard

A. list(map(lambda x: x*2, range(1, 5)))

B. map(lambda x: x**2, [1, 2, 3, 4, 5])

C. list(map(lambda x: x**2, range(1, 6)))

D. list(map(lambda x: x**2, range(5)))

Solution

1. Step 1: Understand the goal

   Create squares of numbers 1 to 5 inclusive, so numbers are 1,2,3,4,5.

2. Step 2: Check each option

   list(map(lambda x: x**2, range(1, 6))) uses range(1,6) which includes 1 to 5 and squares each number correctly.
   list(map(lambda x: x*2, range(1, 5))) doubles numbers and uses range(1,5) which excludes 5.
   map(lambda x: x**2, [1, 2, 3, 4, 5]) returns a map object, not a list.
   list(map(lambda x: x**2, range(5))) uses range(5) which is 0 to 4, not 1 to 5.

3. Final Answer:

   list(map(lambda x: x**2, range(1, 6))) -> Option C

4. Quick Check:

   Squares 1-5 with map and lambda [OK]

Hint: Use range(1,6) and lambda x: x**2 inside list(map()) [OK]

Common Mistakes:

• Using range(5) which starts at 0
• Forgetting to convert map to list
• Using wrong lambda expression like x*2