Adding and updating key-value pairs in Python - Mini Project: Build & Apply

Adding and updating key-value pairs

📖 Scenario: You are managing a small store's inventory. You need to keep track of the products and their quantities using a dictionary.

🎯 Goal: Create a dictionary with initial products and quantities, add a new product, update the quantity of an existing product, and then display the final inventory.

📋 What You'll Learn

Create a dictionary called inventory with exact products and quantities

Add a new product with a specific quantity to inventory

Update the quantity of an existing product in inventory

Print the final inventory dictionary

💡 Why This Matters

🌍 Real World

Managing inventory is a common task in stores and warehouses to keep track of product quantities.

💼 Career

Knowing how to add and update key-value pairs in dictionaries is essential for data management and software development.

1

Create the initial inventory dictionary

Create a dictionary called inventory with these exact entries: 'apples': 10, 'bananas': 5, and 'oranges': 8.

Python

# Create the inventory dictionary with apples, bananas, and oranges
# Your code here

Hint

Use curly braces {} to create a dictionary with keys and values separated by colons.

2

Add a new product to the inventory

Add a new product 'pears' with quantity 12 to the inventory dictionary.

Python

inventory = {'apples': 10, 'bananas': 5, 'oranges': 8}
# Add 'pears' with quantity 12 to inventory
# Your code here

Hint

Use square brackets [] with the new key to add it to the dictionary.

3

Update the quantity of an existing product

Update the quantity of 'bananas' in the inventory dictionary to 7.

Python

inventory = {'apples': 10, 'bananas': 5, 'oranges': 8}
inventory['pears'] = 12
# Update 'bananas' quantity to 7
# Your code here

Hint

Use the key 'bananas' with square brackets to assign the new quantity.

4

Display the final inventory

Print the inventory dictionary to show all products and their quantities.

Python

inventory = {'apples': 10, 'bananas': 5, 'oranges': 8}
inventory['pears'] = 12
inventory['bananas'] = 7
# Print the inventory dictionary
# Your code here

Hint

Use the print() function to display the dictionary.

Practice

1. What happens when you use dictionary[key] = value in Python if the key already exists?

easy

A. A new key-value pair is added without changing the old one.

B. The value for that key is updated to the new value.

C. An error is raised because the key already exists.

D. The dictionary is cleared before adding the new pair.

Solution

1. Step 1: Understand dictionary key assignment

   Using dictionary[key] = value sets the value for the given key.

2. Step 2: Behavior when key exists

   If the key exists, the old value is replaced with the new one.

3. Final Answer:

   The value for that key is updated to the new value. -> Option B

4. Quick Check:

   Assigning existing key updates value [OK]

Hint: Assigning to existing key updates value, no error [OK]

Common Mistakes:

• Thinking it adds a duplicate key
• Expecting an error on existing key
• Assuming dictionary clears automatically

2. Which of the following is the correct syntax to add a new key-value pair 'color': 'blue' to a dictionary named car?

easy

A. car.color = 'blue'

B. car.add('color', 'blue')

C. car('color') = 'blue'

D. car['color'] = 'blue'

Solution

1. Step 1: Recall dictionary syntax for adding pairs

   Use dictionary[key] = value to add or update pairs.

2. Step 2: Check each option

   car['color'] = 'blue' uses correct syntax: car['color'] = 'blue'.

3. Final Answer:

   car['color'] = 'blue' -> Option D

4. Quick Check:

   Use square brackets for keys [OK]

Hint: Use square brackets and equals sign to add/update [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Trying to call add() method which doesn't exist
• Using dot notation which works only for objects

3. What is the output of this code?

fruit_colors = {'apple': 'red', 'banana': 'yellow'}
fruit_colors['banana'] = 'green'
fruit_colors['cherry'] = 'red'
print(fruit_colors)

medium

A. {'apple': 'red', 'banana': 'yellow', 'cherry': 'red'}

B. {'apple': 'red', 'banana': 'green'}

C. {'apple': 'red', 'banana': 'green', 'cherry': 'red'}

D. SyntaxError

Solution

1. Step 1: Update existing key 'banana'

   The value for 'banana' changes from 'yellow' to 'green'.

2. Step 2: Add new key 'cherry'

   The pair 'cherry': 'red' is added to the dictionary.

3. Final Answer:

   {'apple': 'red', 'banana': 'green', 'cherry': 'red'} -> Option C

4. Quick Check:

   Update and add keys correctly [OK]

Hint: Updating key changes value; new key adds pair [OK]

Common Mistakes:

• Assuming old value stays after update
• Forgetting to add new key
• Expecting syntax error from valid code

4. Find the error in this code snippet:

data = {'x': 1, 'y': 2}
data['z'] == 3
print(data)

medium

A. Using '==' instead of '=' to add/update key-value pair.

B. Missing quotes around key 'z'.

C. Dictionary keys must be integers, not strings.

D. print() cannot display dictionaries.

Solution

1. Step 1: Identify the operator used for assignment

   The code uses '==' which is a comparison operator, not assignment.

2. Step 2: Correct operator for adding/updating dictionary

   Use '=' to assign value to a key in dictionary.

3. Final Answer:

   Using '==' instead of '=' to add/update key-value pair. -> Option A

4. Quick Check:

   Use '=' to assign values [OK]

Hint: Use single '=' to assign, not '==' [OK]

Common Mistakes:

• Confusing '==' with '='
• Thinking keys must be numbers
• Believing print can't show dictionaries

5. You have a dictionary inventory = {'apple': 5, 'banana': 3}. You want to add 2 more apples to the count. Which code correctly updates the inventory?

hard

A. All of the above

B. inventory['apple'] += 2

C. inventory.update({'apple': inventory['apple'] + 2})

D. inventory['apple'] = inventory['apple'] + 2

Solution

1. Step 1: Understand ways to update dictionary values

   You can update a value by direct assignment, augmented assignment, or using update() method.

2. Step 2: Check each option

   inventory['apple'] = inventory['apple'] + 2 adds 2 and assigns directly. inventory['apple'] += 2 uses += operator. inventory.update({'apple': inventory['apple'] + 2}) uses update() with new value.

3. Step 3: Confirm all methods work

   All three correctly increase 'apple' count by 2.

4. Final Answer:

   All of the above -> Option A

5. Quick Check:

   Multiple ways to update dictionary values [OK]

Hint: Multiple ways to update values, use any [OK]

Common Mistakes:

• Thinking only one method works
• Forgetting to add current value before assignment
• Using update() incorrectly without new value