Week 8 Day 1 - Documentation & Errors

Lecture/Live Code

There are two parts to today’s class. First we’ll talk about documentation, which is an extremely useful resource that any good programming language has which breaks down some of the

Documentation

You can explore Python’s documentation here. In class, we’ll walk through different components of this page together.

Some things worth exploring here if you are out of class:

“What’s new in Python 3.11?”
Library Reference
Language Reference
Skim through some of the FAQs

We’re also going to specifically look at the itertools module as an example.

When you’re done exploring, go to in-class exercise 1.

Error Handling

Something we haven’t talked about much in class yet is how to deal with input that isn’t what we expect it to be.

assert Statements

Often if there’s an error in our code, it’s because we’re assuming that a variable we’ve created has a different value than what it actually has. Here’s an example.

x = list(range(20))
assert x[-1] == 20

This will give us an assertion error. It can be nice to include a message for such an error:

x = list(range(20))
assert x[-1] == 20 , f"Expected 20, got {x[-1]}"

Go to in-class exercise 2.

You don’t want to use assert statements for checking third-party user input, though; assert statements can be turned off and skipped over entirely, so this isn’t good practice. For that, we instead use try-except.

try-except Clauses

Try-Except is another way of handling errors. In this case, you would have to know what kind of errors you expect to see; however, a benefit of this construction is you can’t turn it off, so the user could see your code, too.

x = input("Please enter a number:")
try:
    x = int(x)
except TypeError:
    print("Not a recognizable number.")

You could also use this to try to open a file.

try:
    with open("baseball.txt") as f:
        print(line for line in f.readlines())
except:
    print("Something went wrong.")

There are two other keywords, else and finally.

Else will execute if no exception occured. Finally will execute no matter what.

try:
    with open("baseball.txt") as f:
        print(line for line in f.readlines())
except:
    print("Something went wrong.")
else:
    print("Nothing went wrong.")
finally:
    print("Done.")

In-Class Exercises

Exercise 1: Random

Find the Python documentation for the random module. See if you can find a way to take the following list and return a new list with the same elements in a random order.

subjects = ["English", "Math", "Science", "Humanities"]

Exercise 2: Assertions

Paste the following code into your python script.

orig_list = [2, 9, 20, 54]
new_list = orig_list
new_list[0] = 11
for x in orig_list:
    if x == 2:
        print(x)

It might not give you the output you’re expecting.

Try adding an assert statement before the for loop to check the first value of the original list. If the assert error fails, add a message to print out what orig_list[0] actually is.