Objective

In this unit, we will learn about list comprehension in Python. List comprehension is a concise way to create lists based on existing lists or other iterable objects. By the end of this unit, you will understand how to use list comprehension and how to incorporate conditionals into it.

Understanding List Comprehension

List comprehension is a syntactic construct available in Python for creating a list based on existing lists. It follows the form of the mathematical set-builder notation. List comprehension can be more readable and efficient than using a for loop to create a list, especially for simple cases.

Here's a simple example of list comprehension:

squares = [x**2 for x in range(10)]
print(squares)

This code creates a list of the squares of the numbers from 0 to 9. The for loop inside the brackets is the same as the one you would use in a normal for loop. The expression before the for keyword is what's done to each element in the original iterable (in this case, squaring the element).

Syntax of List Comprehension

The basic syntax of list comprehension in Python is:

[expression for item in iterable]

• expression is an output expression producing members of the new list from members of the input iterable.
• item is a variable which takes each value in the iterable.
• iterable is a sequence, an iterator, or some other object which supports iteration.

Using Conditionals in List Comprehension

You can also incorporate conditionals into list comprehension to create more complex expressions. Here's an example:

even_squares = [x**2 for x in range(10) if x % 2 == 0]
print(even_squares)

This code creates a list of the squares of the numbers from 0 to 9, but only for the even numbers. The if statement inside the brackets is the same as the one you would use in a normal if statement.

Applications of List Comprehension

List comprehension is a powerful tool that can make your code more efficient and readable when used properly. It's most useful when you need to perform some operation on each element of a list (or other iterable) and create a new list with the results.

Project: Use List Comprehension to Generate a Sequence of Movements for the Turtle

In this project, we will use list comprehension to generate a sequence of movements for the turtle to draw.

import turtle

# Create a new turtle screen and set its background color
screen = turtle.Screen()
screen.setup(width=800, height=600)
screen.bgcolor("white")

# Initialize a turtle object
t = turtle.Turtle()

# Use list comprehension to create a list of movements
movements = [(50, 90) if i % 2 == 0 else (100, 90) for i in range(36)]

# Use a for loop to make the turtle execute the movements
for move in movements:
    t.forward(move[0])
    t.right(move[1])

# Hide the turtle and wait until the window is closed
t.hideturtle()
turtle.done()

In this code, we use list comprehension to create a list of movements for the turtle. Each movement is a tuple, where the first element is the distance the turtle should move forward, and the second element is the angle the turtle should turn right. We use an if statement in the list comprehension to alternate between short and long movements.

Then we use a for loop to make the turtle execute each movement in the list. We use indexing (move[0] and move[1]) to access the elements of each tuple.

Try modifying the list comprehension to create different patterns!

In the next unit, we'll look at ranges, sets, and tuples, and explore how to use these data structures in Python.