Journey of a Python Beginner · Lesson 5 (Part 1) | Dictionaries

Hello everyone, I am Xingyuan, a 19-year-old self-taught Python beginner 🤓. Today, we will unlock the magic of dictionaries! Using key-value pairs to store data, modeling the real world, and let’s tackle the Tic-Tac-Toe project together! 🚀

📌 Today’s Learning Content

👉 “Using dictionaries to store structured data, learning to query, modify, and delete, and modeling a Tic-Tac-Toe board!”

✨ Knowledge Points Explanation

1️⃣ Dictionary Data Type

• Concept: A dictionary is a collection of key-value pairs, represented by <span>{ }</span>. Keys must be unique.

• Example:

  >>> myCat = {'size': 'fat', 'color': 'gray', 'disposition': 'loud'}
  >>> myCat['size']
  'fat'

• Empty Dictionary:<span>{}</span> → No key-value pairs.

2️⃣ Dictionaries vs Lists

• Difference:

• List: Values are stored by integer index, ordered.

• Dictionary: Values are stored by keys, unordered, keys can be of any type (e.g., strings, numbers).

• Example:

  >>> spam = {'name': 'Zophie', 'age': 7}
  >>> spam['color']  # KeyError!

3️⃣ <span>keys()</span>, <span>values()</span>, <span>items()</span> Methods

• Methods:

• <span>keys()</span> → Returns all keys

• <span>values()</span> → Returns all values

• <span>items()</span> → Returns all key-value pairs (in tuple form).

• Example:

  >>> s = {'color': 'black', 'age': 12}
  >>> for i in s.keys():
  ...     print(i)
  ...
  color
  age

4️⃣ Checking Key Existence

• Syntax:

  >>> 'color' in s.keys()
  True
  >>> 'color' in s  # Short form
  True

5️⃣ <span>get()</span> Method

• Function: Safely retrieve a value, returns a default value if the key does not exist.

• Syntax:

  >>> picnicItems = {'apples': 5, 'cups': 2}
  >>> print(picnicItems.get('cups', 0))
  2
  >>> print(picnicItems.get('eggs', 0))
  0

6️⃣ <span>setdefault()</span> Method

• Function: Sets a default value if the key does not exist.

• Syntax:

  >>> s.setdefault('color', 'black')
  'black'
  >>> s
  {'name': 'Xingyuan', 'age': 19, 'color': 'black'}

7️⃣ Pretty Print

• Module:<span>pprint</span> → Formats and prints complex data structures.

• Example:

  import pprint
  
  message = "Python is really amazing"
  count = {}
  for character in message:
      if character not in count:
          count[character] = 0
      count[character] += 1
  print("Character count:")
  pprint.pprint(count)

8️⃣ Tic-Tac-Toe Project

• Goal: Model a Tic-Tac-Toe board using a dictionary and print its state.

• Code:

  game_state = {
      'top-left': ' ', 'top-middle': ' ', 'top-right': ' ',
      'middle-left': ' ', 'middle-middle': ' ', 'middle-right': ' ',
      'bottom-left': ' ', 'bottom-middle': ' ', 'bottom-right': ' '
  }
  
  def display_board(board):
      print(board['top-left'] + '|' + board['top-middle'] + '|' + board['top-right'])
      print('-+-+-')
      print(board['middle-left'] + '|' + board['middle-middle'] + '|' + board['middle-right'])
      print('-+-+-')
      print(board['bottom-left'] + '|' + board['bottom-middle'] + '|' + board['bottom-right'])
  
  display_board(game_state)

✅ Summary

1. Dictionaries are represented by <span>{ }</span>, with key-value pairs separated by commas.

2. Access values using <span>myDict[key]</span>, a missing key will raise a <span>KeyError</span>.

3. <span>keys()</span>, <span>values()</span>, and <span>items()</span> retrieve the keys, values, and key-value pairs of the dictionary, respectively.

4. <span>get()</span> safely retrieves values, while <span>setdefault()</span> sets default values.

5. <span>pprint.pprint()</span> formats and prints dictionaries.

6. Dictionaries can model real-world scenarios, such as a Tic-Tac-Toe board.

📢 Interactive Question

👉 “When you first modeled with dictionaries, did you ever make a mistake in the keys that caused data to get messed up?” Share your “modeling mishap” moments in the comments, and let’s learn from each other!