Lambda syntax and behavior in Python - Mini Project: Build & Apply

Lambda syntax and behavior

📖 Scenario: Imagine you are helping a small shop calculate discounts on products quickly using simple functions.

🎯 Goal: You will create a lambda function to calculate discounted prices, then use it to find the discounted price of a product.

📋 What You'll Learn

Create a lambda function called discount that takes price and rate and returns the discounted price

Use the discount lambda function to calculate the price after a 20% discount on a product priced at 50

Print the discounted price

💡 Why This Matters

🌍 Real World

Shops and businesses often use quick calculations like discounts to update prices fast.

💼 Career

Understanding lambda functions helps you write concise code for small tasks, useful in data processing and automation jobs.

1

Create a lambda function for discount calculation

Create a lambda function called discount that takes two parameters: price and rate. It should return the price after subtracting the discount (price minus price times rate).

Python

# Create the lambda function called discount
# Your code here

Hint

Remember, a lambda function is written as lambda parameters: expression.

2

Set product price and discount rate

Create two variables: product_price set to 50 and discount_rate set to 0.2 (which means 20%).

Python

discount = lambda price, rate: price - price * rate
# Create product_price and discount_rate variables
# Your code here

Hint

Just assign the numbers 50 and 0.2 to the variables as shown.

3

Calculate discounted price using the lambda function

Create a variable called final_price and set it to the result of calling the discount lambda function with product_price and discount_rate as arguments.

Python

discount = lambda price, rate: price - price * rate
product_price = 50
discount_rate = 0.2
# Calculate final_price using discount lambda
# Your code here

Hint

Call the lambda function like a normal function with the two variables.

4

Print the discounted price

Write a print statement to display the value of final_price.

Python

discount = lambda price, rate: price - price * rate
product_price = 50
discount_rate = 0.2
final_price = discount(product_price, discount_rate)
# Print the final_price
# Your code here

Hint

Use print(final_price) to show the discounted price.

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