Bonus Python

Winter 2025

1

Adrian Salguero

Announcements

• Homework 6 is due Friday, March 14th by 11:59pm
• No extensions or resubmissions will be allowed
• Coding Practice 8 is due Wednesday, March 12th by 11:59pm
• Fill out Course Evaluation for the course
• Due by Sunday, March 16th at 11:59pm
• Final Exam is on Thursday March 20th at 2:30pm
• DRS students - register with DRC Testing Center

2

Python Topics for Today

• List comprehensions
• Enumerate a list
• Ternary assignment
• Raising and catching errors

NOTE: None of this material will appear on the Final Exam!

3

List Comprehensions

• Shorter syntactical way to create a new list

list = [expression for item in iterable <if condition == True>]

4

Examples of List Comprehension

list = [x for x in range(4)]

→ [0, 1, 2, 3]

list = [x for x in range(4) if x%2 == 0]

→ [0, 2]

names = [“Vibhav”, “John”, “Suh Young”]

list = [len(x) for x in names]

→ [6, 4, 9]

5

Enumerate a list

Suppose we have the following list:

lst = [10 ** x for x in range (10)]

We can print the values and corresponding index as so:

for i in range(len(lst)):

print(‘value at index’, i, ‘is’, lst[i])

Or we can use the built-in enumerate function:

for index, value in enumerate(lst):

print(‘value at index’, index, ‘is’, value)

6

Enumerate

• Iterating through an enumerate object will print out tuples of the form(index, value)

lst = [100, 200, 300, 400, 500]

for x in enumerate(lst):

print(x)

​

(0, 100)

(1, 200)

(2, 300)

(3, 400)

(4, 500)

​

7

Practice: Enumerate a list

Goal: Write code that creates a list where each element is the sum of the element and it’s index

​

Without list comprehension or enumerate:

lst = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

new_lst = [ ]

for i in range(len(lst)):

new_lst.append(i + lst[i])

8

Practice: Enumerate a list

Goal: Write code that creates a list where each element is the sum of the element and it’s index

​

With list comprehension and enumerate:

lst = [x for x in range(10)]

new_lst = [i + v for i,v in enumerate(lst)]

9

Practice Problem: Enumerate

Given a list of participants, in the order they finished a race, create a dictionary that maps their name to their finishing place.

​

Input

racers = [‘Dino’, ‘Wilma’, ‘Barney’, ‘Fred’]

​

Output

race_dict = {‘Dino’: 1, ‘Wilma’: 2, ‘Barney’: 3, ‘Fred’: 4}

10

Ternary Assignment

A common pattern in Python code

if x > threshold:

flag = “Over”

else:

flag = “Under”

​

With a ternary expression

flag = “Over” if x > threshold else “Under”

11

Other Forms of Ternary Assignments

If-elif-else:

if temperature > 90:

weather = “Hot”

elif temperature < 40:

weather = “Cold”

else:

weather = “Mild”

​

With ternary assignments

weather = “Hot” if temperature > 90 else “Cold” if temperature < 40 else “Mild”

12

Other Forms of Ternary Assignments

With Tuples:

flag = (“Negative”, “Positive”)[x >= 0]

​

With Dictionaries:

flag = {True: “Positive”, False: “Negative”} [x >= 0]

13

Other Forms of Ternary Assignments

With Tuples:

flag = (false_value, true_value)[x >= 0]

​

With Dictionaries:

flag = {True: true_value, False: false_value} [x >= 0]

14

Raising (“Throwing”) Errors

• Suppose you want to raise (or throw) an error in your function in the case that a user incorrectly uses the function.

def always_raises_error():

raise ValueError(“This is a value error!”)

15

Catching Errors

• You can also “catch” and error and provide additional information about the error.

​

def always_raises_error():

raise ValueError(“This is a value error!”)

​

try:

always_raises_error()

except ValueError:

print(“Please don’t call this function!”)

16

Trying and Catching IndexOutOfBounds

def print_val_at_index(lst, index):

try:

lst[index]

except IndexError:

print("Index out of bounds!")

else:

print(lst[index])

​

17

Common Errors

• IndexError – Index out of range
• KeyError – Key not found in dictionary
• IndentationError – Invalid indentation
• TypeError – Operation applied to invalid combination of types
• ValueError – Function gets properly typed argument, but invalid value
• SyntaxError – Invalid Python syntax
• NameError – Variable name not found
• FloatingPointError – Floating point operation fails
• RuntimeError – Otherwise Unknown Error

​

18