Accessing values using keys in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a key in a Python dictionary?

A key is a unique identifier used to access a value in a Python dictionary. Think of it like a label on a box that helps you find what's inside.

beginner

How do you access a value using a key in a Python dictionary?

You use square brackets with the key inside, like dictionary[key]. This tells Python to find the value linked to that key.

beginner

What happens if you try to access a key that does not exist in a dictionary?

Python will give a KeyError. It's like asking for a box that isn't there.

intermediate

How can you safely access a value for a key that might not exist?

Use the get() method, like dictionary.get(key, default). It returns the value if the key exists, or a default value if it doesn't.

beginner

Example: Given person = {'name': 'Alice', 'age': 30}, how do you get Alice's age?

Use person['age']. This will give you 30, the value linked to the key 'age'.

What will my_dict['color'] return if my_dict = {'color': 'blue'}?

A None

B 'blue'

C KeyError

D 'color'

Accessing the key 'color' returns its value 'blue'.

What error occurs if you access a missing key in a dictionary using square brackets?

A TypeError

B ValueError

C KeyError

D IndexError

Trying to access a key that doesn't exist raises a KeyError.

Which method allows safe access to a dictionary key with a default value?

A dictionary.get()

B dictionary.find()

C dictionary.access()

D dictionary.lookup()

The get() method returns the value or a default if the key is missing.

If data = {'x': 10}, what does data.get('y', 0) return?

A KeyError

B None

C 10

D 0

Since 'y' is missing, get() returns the default value 0.

How do you access the value for key 'name' in info = {'name': 'Bob'}?

A info['name']

B info.get_name()

C info.name

D info.get('age')

Use square brackets with the key string: info['name'].

Explain how to access a value in a Python dictionary using a key.

Think about how you find a labeled box in a shelf.

Describe how to safely get a value from a dictionary when the key might not exist.

It's like asking politely and getting a backup answer.

Practice

1. What is the correct way to access the value associated with the key 'name' in the dictionary person = {'name': 'Alice', 'age': 30}?

easy

A. person.get('age')

B. person.name

C. person['name']

D. person['Alice']

Solution

1. Step 1: Identify the dictionary and key

   The dictionary is person and the key to access is 'name'.

2. Step 2: Use correct syntax to access value by key

   In Python, dictionary values are accessed using square brackets and the key as a string: person['name'].

3. Final Answer:

   person['name'] -> Option C

4. Quick Check:

   Access value by key = person['name'] [OK]

Hint: Use square brackets with the key string to get value [OK]

Common Mistakes:

• Using dot notation like person.name (not valid for dict)
• Using a value instead of key inside brackets
• Accessing a different key than asked

2. Which of the following is the correct syntax to safely get the value for key 'city' from dictionary data without causing an error if the key does not exist?

easy

A. data['city']

B. data.get('city')

C. data.city

D. data['City']

Solution

1. Step 1: Understand safe access in dictionaries

   Using data['city'] causes an error if the key is missing. Using data.get('city') returns None instead.

2. Step 2: Identify correct method for safe access

   The get() method is designed to safely access keys without errors.

3. Final Answer:

   data.get('city') -> Option B

4. Quick Check:

   Safe key access = data.get('city') [OK]

Hint: Use get() to avoid errors if key missing [OK]

Common Mistakes:

• Using square brackets which raise KeyError if key missing
• Using dot notation which is invalid for dict
• Using wrong key case causing KeyError

3. What will be the output of this code?

info = {'a': 1, 'b': 2, 'c': 3}
print(info['b'])

medium

A. 2

B. 1

C. 3

D. KeyError

Solution

1. Step 1: Identify the dictionary and key accessed

   The dictionary info has key 'b' with value 2.

2. Step 2: Access the value using the key

   The code prints info['b'], which is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   info['b'] = 2 [OK]

Hint: Look up the key's value directly in the dictionary [OK]

Common Mistakes:

• Confusing key with value
• Expecting KeyError when key exists
• Mixing up keys and values

4. The following code causes an error. What is the problem?

data = {'x': 10, 'y': 20}
print(data['z'])

medium

A. TypeError because keys must be integers

B. SyntaxError due to wrong brackets

C. No error, prints 0

D. KeyError because 'z' is not in the dictionary

Solution

1. Step 1: Check if key exists in dictionary

   The key 'z' is not present in data.

2. Step 2: Understand error caused by missing key

   Accessing a missing key with square brackets raises a KeyError.

3. Final Answer:

   KeyError because 'z' is not in the dictionary -> Option D

4. Quick Check:

   Missing key access = KeyError [OK]

Hint: Check if key exists before accessing or use get() [OK]

Common Mistakes:

• Assuming missing keys return 0 or None
• Confusing syntax error with runtime error
• Using wrong bracket types

5. Given the dictionary grades = {'Alice': 85, 'Bob': 92, 'Charlie': 78}, which code snippet correctly prints Bob's grade or 'Not found' if Bob is not in the dictionary?

hard

A. print(grades.get('Bob', 'Not found'))

B. print(grades['Bob'] or 'Not found')

C. print(grades['bob'] or 'Not found')

D. print(grades.get('Bob'))

Solution

1. Step 1: Understand the goal

   We want to print Bob's grade if present, otherwise print 'Not found'.

2. Step 2: Analyze each option

   print(grades['Bob'] or 'Not found') will raise KeyError if key missing because access happens before 'or'. print(grades.get('Bob', 'Not found')) uses get() with default value, which is concise and safe. print(grades['bob'] or 'Not found') uses wrong key case and will fail. print(grades.get('Bob')) prints None if key missing, not 'Not found'.

3. Final Answer:

   print(grades.get('Bob', 'Not found')) -> Option A

4. Quick Check:

   Use get() with default for safe access [OK]

Hint: Use get(key, default) to handle missing keys easily [OK]

Common Mistakes:

• Using wrong key case causing missing key
• Not providing default value in get()
• Assuming or operator works for missing keys