day3_cheatsheet.md

🐍 Python Day 3 Cheat Sheet

Storing Collections of Data

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📝 Table of Contents

1. Core Definitions
2. Key Differences at a Glance
3. Lists
4. Dictionaries
5. Tuples
6. Sets
7. Quick Reference
8. Pro Tips
9. Common Mistakes

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🧠 Core Definitions

• List: An ordered and changeable collection that allows duplicate members. It's your most common tool for storing a sequence of items.
• Dictionary: A collection of key-value pairs. It's changeable and does not allow duplicate keys. Used for connecting related pieces of information.
• Tuple: An ordered and unchangeable collection that allows duplicate members. Use it for data that you know should not change.
• Set: An unordered and unindexed collection that does not allow duplicate members. Use it to store unique items and perform mathematical set operations.

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📊 Key Differences at a Glance

Feature	List	Dictionary	Tuple	Set
Syntax	[ ]	{key: value}	( )	{item1, item2}
Ordered	Yes	Yes (in Python 3.7+)*	Yes	No
Changeable	Yes	Yes	No	Yes
Duplicates	Yes	No (Keys)	Yes	No
Access	Index	Key	Index	N/A

*While dictionaries are ordered in modern Python, it's best practice to access items by key, not by their position.

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🗂️ Lists

Creating & Accessing

# Create a list with square brackets []
inventory = ["sword", "shield", "potion"]

# Access by index (starts at 0)
first_item = inventory[0]  # "sword"
last_item = inventory[-1] # "potion"

Slicing (Getting Multiple Items)

# Syntax: list[start:stop]
# Grabs items from start up to (but not including) stop
first_two = inventory[0:2] # ["sword", "shield"]
all_but_first = inventory[1:] # ["shield", "potion"]

Adding Items

# .append() adds to the end
inventory.append("gold coin")
# inventory is now ["sword", "shield", "potion", "gold coin"]

# .insert() adds at a specific index
inventory.insert(1, "armor")
# inventory is now ["sword", "armor", "shield", "potion", "gold coin"]

Removing Items

# del removes by index
del inventory[2] # Removes "shield"

# .remove() searches for and removes a value
inventory.remove("potion")

# .pop() removes the last item and returns it
removed_item = inventory.pop() # removed_item is "gold coin"

Key Point: Lists are ordered, changeable, and your go-to for storing a sequence of items.

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📖 Dictionaries

Creating & Accessing

# Create a dictionary with curly braces {} and key:value pairs
player_stats = {
    "name": "Alex",
    "level": 5,
    "health": 100
}

# Access values by their key (not by index!)
player_name = player_stats["name"] # "Alex"
player_health = player_stats["health"] # 100

Adding & Modifying

# Add a new key-value pair
player_stats["mana"] = 50

# Modify an existing value
player_stats["level"] = 6

Removing Items

# del removes a key-value pair by its key
del player_stats["health"]

Key Point: Dictionaries are for storing connected data. The order is not guaranteed (in older Python versions), but the link between a key and its value is.

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📦 Tuples

Creating & Accessing

# Create a tuple with parentheses ()
# Tuples are IMMUTABLE (cannot be changed)
coordinates = (100, 250)
rgb_color = (255, 0, 0)

# Access just like a list
x_coord = coordinates[0] # 100

What You CAN'T Do

# These lines will cause an ERROR
coordinates[0] = 150      # Cannot change a value
coordinates.append(50)    # Cannot add a value
del coordinates[0]        # Cannot remove a value

Key Point: Use tuples for data that should never change, like coordinates or fixed settings.

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✨ Sets

Creating & Using

# Create a set from a list to get unique items
tags = ["python", "code", "fun", "python", "guide"]
unique_tags = set(tags)
# unique_tags is {"python", "code", "fun", "guide"}

# Sets are UNORDERED. You cannot access by index.
# The main use is checking for membership.
has_python = "python" in unique_tags # True
has_java = "java" in unique_tags   # False

Key Point: Use sets when you need to store unique items and quickly check if an item exists in the collection.

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⚡ Quick Reference

List Methods

my_list.append(item)    # Add to end
my_list.insert(i, item) # Add at index i
my_list.remove(item)    # Remove first occurrence of item
my_list.pop(i)          # Remove and return item at index i (or last item if i is omitted)
len(my_list)            # Get number of items

Dictionary Methods

my_dict[key] = value    # Add or modify entry
del my_dict[key]        # Remove entry
key in my_dict          # Check if key exists
len(my_dict)            # Get number of key-value pairs

General Functions

# Works on lists, dictionaries, tuples, and sets
len(collection)

# Create from another collection
list(my_tuple)
set(my_list)

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💡 Pro Tips

1. Choose the Right Tool:

   • Need an ordered collection you can change? Use a List.
   • Need to link keys to values? Use a Dictionary.
   • Need an ordered collection that never changes? Use a Tuple.
   • Need to store unique items and check for membership? Use a Set.

2. Combine Data Structures: You can nest collections inside each other for powerful data organization.

   # A list of dictionaries
   players = [
       {"name": "Alice", "level": 10},
       {"name": "Bob", "level": 8}
   ]
   # Accessing nested data
   print(players[0]["name"]) # "Alice"

3. Check Before Accessing: Use the in keyword to avoid errors when accessing dictionary keys.

   if "mana" in player_stats:
       print(f"Mana: {player_stats['mana']}")
   else:
       print("This player has no mana.")

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🎯 Common Mistakes to Avoid

❌ Trying to access a dictionary by index.

# Wrong
first_stat = player_stats[0] # This will cause a KeyError!

✅ Access dictionaries by key.

# Correct
first_stat = player_stats["name"]

❌ Forgetting quotes around string keys in a dictionary.

# Wrong
stats = {name: "Alex"} # This will cause a NameError!

✅ Always use quotes for string keys.

# Correct
stats = {"name": "Alex"}

❌ Trying to change a tuple.

# Wrong
my_tuple = (1, 2, 3)
my_tuple[0] = 99 # This will cause a TypeError!

✅ If you need to change it, make it a list first.

# Correct
my_list = list(my_tuple)
my_list[0] = 99
my_tuple = tuple(my_list) # Optional: convert back to tuple

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Happy coding! 🐍✨