Accessing values using keys in Python - Step-by-Step Execution

Concept Flow - Accessing values using keys

Start with dictionary

↓

Provide key to access

↓

Check if key exists

↓

Return value

↓

End

This flow shows how a dictionary value is accessed by a key: check if the key exists, then return the value or handle missing key.

Execution Sample

Python

my_dict = {'apple': 5, 'banana': 3}
value = my_dict['apple']
print(value)

This code gets the value for key 'apple' from the dictionary and prints it.

Execution Table

Step	Action	Key Provided	Key Exists?	Value Retrieved	Output
1	Create dictionary	-	-	{'apple': 5, 'banana': 3}	-
2	Access value	'apple'	Yes	5	-
3	Print value	-	-	-	5

💡 Finished accessing and printing the value for key 'apple'.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
my_dict	undefined	{'apple': 5, 'banana': 3}	{'apple': 5, 'banana': 3}	{'apple': 5, 'banana': 3}
value	undefined	undefined	5	5

Key Moments - 2 Insights

What happens if the key does not exist in the dictionary?

If the key is missing, Python raises a KeyError. This is shown by the 'Key Exists?' check in the flow. To avoid errors, use methods like dict.get() or check key presence before access.

Why do we use square brackets [] to access dictionary values?

Square brackets tell Python to look up the value for the given key in the dictionary. This is shown in Step 2 where 'apple' is inside [].

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'value' after Step 2?

A undefined

B 3

C 5

D KeyError

💡 Hint

Check the 'Value Retrieved' column at Step 2 in the execution_table.

At which step is the dictionary created?

A Step 2

B Step 1

C Step 3

D After Step 3

💡 Hint

Look at the 'Action' column in the execution_table for when the dictionary is created.

If we try to access my_dict['orange'], what would happen?

A Raise KeyError

B Return 0

C Return None

D Return 'orange'

💡 Hint

Recall the 'Key Exists?' decision in the concept_flow and what happens if key is missing.

Concept Snapshot

Access dictionary values using keys with square brackets: value = dict[key]
If key exists, returns the value; else raises KeyError.
Use dict.get(key) to avoid errors and get None or default.
Keys must be immutable types like strings or numbers.
Accessing values is fast and direct.

Full Transcript

This lesson shows how to get values from a dictionary using keys. We start with a dictionary that stores pairs of keys and values. To get a value, we provide the key inside square brackets. Python checks if the key exists. If yes, it returns the value. If not, it raises an error. The example code creates a dictionary with fruits and counts, then gets the count for 'apple' and prints it. Variables change as the dictionary is created and the value is retrieved. Beginners often wonder what happens if the key is missing or why square brackets are used. The quiz asks about variable values at steps and what happens with missing keys. Remember, use dict.get() to safely access keys that might not exist.

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