Adding and updating key-value pairs in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you add a new key-value pair to a Python dictionary?

You add a new key-value pair by assigning a value to a new key using square brackets. For example: my_dict['new_key'] = 'new_value'.

beginner

What happens if you assign a value to an existing key in a Python dictionary?

The existing key's value is updated (replaced) with the new value. For example, my_dict['key'] = 'new_value' changes the old value to 'new_value'.

intermediate

Show how to add multiple key-value pairs to a dictionary at once.

Use the update() method with another dictionary or iterable of pairs. Example: my_dict.update({'a': 1, 'b': 2}) adds or updates keys 'a' and 'b'.

intermediate

Can you use the setdefault() method to add a key-value pair? How?

Yes. setdefault(key, default_value) adds the key with default_value if the key is not present. If the key exists, it returns its value without changing it.

intermediate

What is the difference between adding a key-value pair using assignment and using update()?

Assignment adds or updates a single key-value pair: my_dict[key] = value. update() can add or update multiple pairs at once from another dictionary or iterable.

What does this code do?
my_dict['color'] = 'blue'

A Adds or updates the key 'color' with value 'blue'

B Deletes the key 'color'

C Checks if 'color' is in the dictionary

D Creates a new dictionary

Assigning a value to a key adds it if missing or updates it if present.

Which method adds multiple key-value pairs to a dictionary at once?

A append()

B update()

C insert()

D add()

update() merges another dictionary or iterable of pairs into the dictionary.

What does setdefault('key', 0) do if 'key' is already in the dictionary?

A Updates 'key' to 0

B Removes 'key'

C Returns the current value of 'key' without changing it

D Raises an error

setdefault returns the existing value if the key exists.

If you want to change the value of an existing key, what should you do?

A Assign a new value to the key using <code>[]</code>

B Use <code>del</code> then add new key

C Use <code>pop()</code>

D Use <code>clear()</code>

Assigning a new value to the key updates it directly.

What will this code do?
my_dict.update({'x': 10, 'y': 20})

A Create a new dictionary with keys 'x' and 'y'

B Delete keys 'x' and 'y'

C Raise an error

D Add or update keys 'x' and 'y' with values 10 and 20

update() adds or updates multiple key-value pairs.

Explain how to add a new key-value pair and update an existing key in a Python dictionary.

Think about using square brackets with the key.

Describe the difference between using assignment and the update() method to add or update dictionary entries.

One is for one pair, the other can handle many.

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