09 - DICTIONARIES

Study Guide for Python Programming: Unit on Dictionaries

Here is a Dictionary of dog emotions:

{“Boris”: “mild confusion”, “Bert”: “cape fueled rage”}

List Syntax

Boris made a summary of the syntax for each of the Python dictionary functions in the study guide:

• keys()
  • Returns a view object displaying a list of all the keys in the dictionary.
  • Syntax: dictionary.keys()
• values()
  • Returns a view object containing a list of all the values in the dictionary.
  • Syntax: dictionary.values()
• get()
  • Returns the value for the specified key if the key is in the dictionary.
  • Syntax: dictionary.get(key, default)
  • The default parameter is optional and provides a value to return if the key is not found. If not provided, it defaults to None.

Key Concepts

Basic Concepts of Dictionaries

• Key-Value Pairs: Dictionaries store data as key-value pairs.
• Mutability: Python dictionaries are mutable, meaning their values can be changed after creation.
• Accessing Values: Values in a dictionary are accessed via keys, not indices (e.g., dict[key]).

Basic Dictionary Operations

• Creating a Dictionary: Dictionaries are created with curly braces {} or the dict() constructor.
• Updating Values: Change the value of an item by referencing its key (e.g., dict[key] = new_value).
• Handling KeyError: Occurs when a key is not found in the dictionary.

Dictionary Methods

• Useful Methods: get(), keys(), values(), del for deleting a key.
• Handling KeyError with get(): Avoids an error by returning None if the key is not found.

Lists vs. Dictionaries

• When to Use: Lists are for collections of similar items; dictionaries are for storing records with different types of information.

Lists of Dictionaries

• Representing Complex Data Structures: Useful for storing multiple records (e.g., a list of student records).
• Accessing Data: Chaining indices and keys (e.g., students[0]['Name']).

JSON and Dictionaries

• JSON: A standard format for data interchange, closely resembling Python dictionaries.
• Serializing and Deserializing: Converting between JSON strings and Python dictionaries.
• Python’s json Module: Used for loading and saving dictionaries to and from JSON.

Practice Questions

6. What is the value returned by font.get('Size') if font is a dictionary defined as font = {'Name': 'Arial'}?

   • A. None
   • B. KeyError
   • C. 0
   • D. 'Arial'

7. What does the following code do? x['new_key'] = 'new_value' assuming x is a dictionary?

   • A. Throws an error because ‘new_key’ does not exist.
   • B. Adds a new key-value pair to the dictionary.
   • C. Changes the value of an existing key.
   • D. Does nothing to the dictionary.

8. If student = {'Name': 'John', 'Age': 21}, what will student['Age'] = 22 do?

   • A. Adds a new key ‘Age’ with the value 22.
   • B. Changes the value of the ‘Age’ key to 22.
   • C. Throws a KeyError.
   • D. Creates a new dictionary with ‘Age’ set to 22.

9. In the context of dictionaries, what does the del keyword do?

   • A. Deletes the entire dictionary.
   • B. Deletes a specified key-value pair.
   • C. Deletes the value of a specified key.
   • D. Clears all values leaving an empty dictionary.

10. What is the result of len(s) where s = [{'a': 1}, {'b': 2}, {'c': 3}]?

    • A. 3
    • B. 6
    • C. 2
    • D. 9

11. What is the output of the following code?

    x = {'a': 5, 'b': {'c': 2, 'd': 4}}
    print(x['b']['d'])

    A. 5 B. {'c': 2, 'd': 4} C. 2 D. 4

12. What is the output of the following code?

    x = {'a': [1, 2], 'b': [3, 4]}
    x['a'].append(3)
    print(x['a'])

    A. [1, 2] B. [1, 2, 3] C. [3, 4, 3] D. KeyError

13. What is the output of the following code?

    x = {'a': 'hello', 'b': 'world'}
    print('a' in x)

    A. True B. False C. 'hello' D. KeyError

14. What happens when the following code is executed?

    x = {'a': 1, 'b': 2}
    del x['a']
    print(x)

    A. {'a': 1} B. {'b': 2} C. {1, 2} D. KeyError

15. What is the result of the following code?

    x = {'a': [1, 2, 3], 'b': [4, 5, 6]}
    print(len(x))

    A. 2 B. 3 C. 6 D. KeyError

Correct Answers and Explanations

6. Answer: A. None
   • Explanation: The get() method returns None if the key is not found in the dictionary.
7. Answer: B. Adds a new key-value pair to the dictionary.
   • Explanation: If the key does not exist, it adds the key-value pair to the dictionary.
8. Answer: B. Changes the value of the ‘Age’ key to 22.
   • Explanation: Since the ‘Age’ key exists, it updates its value to 22.
9. Answer: B. Deletes a specified key-value pair.
   • Explanation: The del keyword removes a specific key-value pair from a dictionary.
10. Answer: A. 3
    • Explanation: The len() function returns the number of items (dictionaries in this case) in the list s, which is 3.
11. Answer: D. 4
    • Explanation: The code retrieves the value of key ‘d’ within the nested dictionary associated with key ‘b’.
12. Answer: B. [1, 2, 3]
    • Explanation: The append() method adds the element 3 to the list associated with key ‘a’.
13. Answer: A. True
    • Explanation: The in operator checks if ‘a’ is a key in the dictionary x, which it is.
14. Answer: B. {'b': 2}
    • Explanation: The del statement removes the key ‘a’ and its value from the dictionary, leaving only {'b': 2}.
15. Answer: A. 2
    • Explanation: The len() function returns the number of key-value pairs in the dictionary x, which is 2.