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Activity 3.4 List Functions and Keywords

Introduction

Python provides a number of different useful functions and keywords for working with lists. We will learn a small subset of functions, but be sure to check out the documentation at the end of this activity for a full list of included functions.

Examples

The in Keyword - Check if an element exists

The in keyword was introduced back when we learned about the for loop. It can be used in different contexts. For example, it’s important to be able to check if an element exists in a list. Here’s how you can use the in keyword to do that:

stuff = ["Xbox", "Playstation", "Nintendo"]

if "Atari" in stuff:
    print("Playing the Atari")
else:
    print("I don't have that console :(")

.append(new_element)

Appends, or adds, an element at the end of a list.

chips = ["jalapeno", "sour cream", "barbeque", "cheddar", "cool ranch"]
chips.append("salt & vinegar")
chips.append("nacho cheese")
print(chips)

["jalapeno", "sour cream", "barbeque", "cheddar", "cool ranch", "salt & vinegar", "nacho cheese"]

.count(value)

Returns the number of times the value was found in the list.

chips = ["jalapeno", "sour cream", "barbeque", "cheddar", "cool ranch"]
print(chips.count('cheddar'))
print(chips.count('hawaiian bbq'))

1
0

.remove(element)

Attempts to remove an element matching the value of the argument. If the element is not found, a ValueError will result. Hint: Before attempting to remove an element, consider first checking if it exists using the in keyword.

animals = ["mongoose", "camel", "wolf", "sheep"]
# If camel was not in the list, you would get this error: ValueError: list.remove(x): x not in list
animals.remove("camel")
print(animals)

["mongoose", "wolf", "sheep"]

.clear()

Removes all elements from the list.

animals = ["mongoose", "camel", "wolf", "sheep"]
print(animals)
animals.clear()
print(animals)

["mongoose", "camel", "wolf", "sheep"]
[]

Skills to Practice

• Adding elements to a list using the .append() function.
• Finding how many elements exist in a list using the .count() function.
• Removing an element from a list using the .remove() function.
• Removing all elements from a list using the .clear() function.

Instructions

Setup

1. Create a new repl.
   1. Select Python for the language.
   2. Name the Repl ”3.4 List Functions“.
2. Click Create repl.

Code

cart = []
store_items = ["potion", "sword", "axe", "scimitar"]
shopping = True

while shopping:
    print("-- ⚗️🧪Store Menu🗡️🪓 --")
    print("1) Add item to cart")
    print("2) List cart contents")
    print("3) Remove an item from cart")
    print("4) Remove all items from cart")
    print("5) Exit")
    choice = input("-> ")

    if choice == "1":
        print("Available items:")

        for item in store_items:
            print(f"- {item}")

        choice = input("Choose item: ")

        if choice in store_items:
            print(f"Adding {choice} to cart\n")
            cart.append(choice)
        else:
            print("Invalid item...\n")
    elif choice == "2":
        print("Cart contents:")

        for item in cart:
            print(f"- {item}")

        print()
    elif choice == "3":
        choice = input("Choose item to remove: ")

        if choice in store_items:
            cart.remove(choice)
            print(f"{choice} removed\n")
        else:
            print("Invalid item...\n")
    elif choice == "4":
        print("Clearing cart contents\n")
        cart.clear()
    elif choice == "5":
        print("Thank you for shopping with us.")
        shopping = False
    else:
        print("Invalid option. Try again.\n")

Debug

-- ⚗️🧪Store Menu🗡️🪓 --
1) Add item to cart
2) List cart contents
3) Remove an item from cart
4) Remove all items from cart
5) Exit
-> 1
Available items:
- potion
- sword
- axe
- scimitar
Choose item: axe
Adding axe to cart

-- ⚗️🧪Store Menu🗡️🪓 --
1) Add item to cart
2) List cart contents
3) Remove an item from cart
4) Remove all items from cart
5) Exit
-> 1
Available items:
- potion
- sword
- axe
- scimitar
Choose item: potion
Adding potion to cart

-- ⚗️🧪Store Menu🗡️🪓 --
1) Add item to cart
2) List cart contents
3) Remove an item from cart
4) Remove all items from cart
5) Exit
-> 2
Cart contents:
- axe
- potion

-- ⚗️🧪Store Menu🗡️🪓 --
1) Add item to cart
2) List cart contents
3) Remove an item from cart
4) Remove all items from cart
5) Exit
-> 4
Clearing cart contents

-- ⚗️🧪Store Menu🗡️🪓 --
1) Add item to cart
2) List cart contents
3) Remove an item from cart
4) Remove all items from cart
5) Exit
-> 2
Cart contents:

Tips, Tricks, and Reflection

• Note that the Python documentation refers to these functions as methods. Methods are functions that belong to a given data type. For now, don’t worry about these semantics, we will cover these topics soon.
• Check out the documentation on the available functions here.