Lecture 6: Conditionals and Exceptions

CSCI 1360E: Foundations for Informatics and Analytics

Overview and Objectives

In this lecture, we'll go over how to make "decisions" over the course of your code depending on the values certain variables take. We'll also introduce exceptions and how to handle them gracefully. By the end of the lecture, you should be able to

• Build arbitrary conditional hierarchies to test a variety of possible circumstances
• Construct elementary boolean logic statements
• Catch basic errors and present meaningful error messages in lieu of a Python crash

Part 1: Conditionals

Up until now, we've been somewhat hobbled in our coding prowess; we've lacked the tools to make different decisions depending on the values our variables take.

For example: how do you find the maximum value in a list of numbers?

x = [51, 65, 56, 19, 11, 49, 81, 59, 45, 73]

If we want to figure out the maximum value, we'll obviously need a loop to check each element of the list (which we know how to do), and a variable to store the maximum.

max_val = 0
for element in x:
    
    # ... now what?

    pass

We also know we can check relative values, like max_val < element. If this evaluates to True, we know we've found a number in the list that's bigger than our current candidate for maximum value. But how do we execute code until this condition, and this condition alone?

Enter if / elif / else statements! (otherwise known as "conditionals")

We can use the keyword if, followed by a statement that evaluates to either True or False, to determine whether or not to execute the code. An example:

x = 5
if x < 5:
    print("How did this happen?!")  # Spoiler alert: this won't happen.

if x == 5:
    print("Working as intended.")

Working as intended.

In conjunction with if, we also have an else clause that we can use to execute whenever the if statement doesn't:

x = 5
if x < 5:
    print("How did this happen?!")  # Spoiler alert: this won't happen.
else:
    print("Correct.")

Correct.

This is great! We can finally finish computing the maximum element of a list!

x = [51, 65, 56, 19, 11, 49, 81, 59, 45, 73]
max_val = 0  # Set a "container" for our maximum.
for element in x:  # Loop through the list.
    if max_val < element:
        max_val = element

print("The maximum element is: {}".format(max_val))

The maximum element is: 81

Let's pause here and walk through that code.

x = [51, 65, 56, 19, 11, 49, 81, 59, 45, 73]

• This code defines the list we want to look at.

max_val = 0

• And this is a placeholder for the eventual maximum value (we need a placeholder like this because of how loops work: in a loop, we can only look at one number at a time, so on each step of the loop, we update our current "guess" of the maximum).

for element in x:

• A standard for loop header: we're iterating over the list x, with the value from the list in the current loop stored in the variable element.

if max_val < element:

• The first line of the loop body is an if statement. This statement asks: is the value in our current max_val placeholder smaller than the element of the list stored in element?

max_val = element

• If the answer to that if statement is True, then this line executes: it sets our placeholder equal to the current list element.

Another way to examine this loop would be to walk through the list x itself.

• On the first iteration, max_val = 0 and element = 51. The conditional is: if 0 < 51. This is True! So we set max_val = 51. That ends the first iteration.

• On the second iteration, max_val = 51 and element = 65. The conditional is: if 51 < 65. This is True! So we set max_val = 65. That ends the second iteration.

• On the third iteration, max_val = 65 and element = 56. The conditional is: if 65 < 56. This is False, so the statement underneath doesn't execute, and nothing else happens in the third iteration.

This process continues for every value in the list. Once the final element of x is evaluated this way, the loop will end, and whatever the value of the variable max_val is will be the maximum value of the list.

Let's look at slightly more complicated but utterly classic example: assigning letter grades from numerical grades.

student_grades = {
    'Jen': 82,
    'Shannon': 75,
    'Natasha': 94,
    'Benjamin': 48,
}

We know the 90-100 range is an "A", 80-89 is a "B", and so on. How would we build a conditional to assign letter grades?

The third and final component of conditionals is the elif statement (short for "else if").

elif allows us to evaluate as many options as we'd like, all within the same conditional context (this is important). So for our grading example, it might look like this:

letter = ''

# Remember the .items() function for dictionaries? Here it is!
# This loop goes over the dictionary, pulling out each key/value
# pair, one at a time, storing them in student/grade.
for student, grade in student_grades.items():
    if grade >= 90:
        letter = "A"
    elif grade >= 80:
        letter = "B"
    elif grade >= 70:
        letter = "C"
    elif grade >= 60:
        letter = "D"
    else:
        letter = "F"
    
    print(student, letter)

Benjamin F
Jen B
Natasha A
Shannon C

Ok, that's neat. But there's still one more edge case: what happens if we want to enforce multiple conditions simultaneously?

To illustrate, let's go back to our example of finding the maximum value in a list, and this time, let's try to find the second-largest value in the list. For simplicity, let's say we've already found the largest value.

x = [51, 65, 56, 19, 11, 49, 81, 59, 45, 73]
max_val = 81  # We've already found it!
second_largest = 0

Here's the rub: we now have two constraints to enforce--the second largest value needs to be larger than pretty much everything in the list, but also needs to be smaller than the maximum value. Not something we can encode using if / elif / else.

Instead, we'll use two more keywords integral to conditionals: and and or.

You've already seen and: this is used to join multiple boolean statements together in such a way that, if one of the statements is False, the entire statement is False.

# An example where only 1 thing is False -- makes the whole thing False.
True and True and True and True and True and True and False

False

One False ruins the whole thing.

However, we haven't encountered or before. How do you think it works?

Here's are two examples:

True or True or True or True or True or True or False

True

False or False or False or False or False or False or True

True

Figured it out?

Where and needs every statement it joins to be True in order for the whole statement to be True, only one statement among those joined by or needs to be True for everything to be True.

How about this example?

(True and False) or (True or False)

True

Stop here for a bit and convince yourself of what is happening. Ask questions if you're confused!

(Order of operations works the same way!)

Getting back to conditionals, then: we can use this boolean logic to enforce multiple constraints simultaneously.

for element in x:
    if second_largest < element and element < max_val:
        second_largest = element

print("The second-largest element is: {}".format(second_largest))

The second-largest element is: 73

Let's step through the code.

for element in x:
    if second_largest < element and element < max_val:
        second_largest = element

• The first condition, second_largest < element, is the same as before: if our current estimate of the second largest element is smaller than the latest element we're looking at, it's definitely a candidate for second-largest.

• The second condition, element < max_val, is what ensures we don't just pick the largest value again. This enforces the constraint that the current element we're looking at is also less than the maximum value.

• The and keyword glues these two conditions together: it requires that they BOTH be True before the code inside the statement is allowed to execute.

It would be easy to replicate this with "nested" conditionals:

second_largest = 0
for element in x:
    if second_largest < element:
        if element < max_val:
            second_largest = element

print("The second-largest element is: {}".format(second_largest))

The second-largest element is: 73

...but your code starts getting a little unwieldy with so many indentations.

You can glue as many comparisons as you want together with and; the whole statement will only be True if every single condition evaluates to True. This is what and means: everything must be True.

The other side of this coin is or. Like and, you can use it to glue together multiple constraints. Unlike and, the whole statement will evaluate to True as long as at least ONE condition is True. This is far less stringent than and, where ALL conditions had to be True.

numbers = [1, 2, 5, 6, 7, 9, 10]
for num in numbers:
    if num == 2 or num == 4 or num == 6 or num == 8 or num == 10:
        print("{} is an even number.".format(num))

2 is an even number.
6 is an even number.
10 is an even number.

In this contrived example, I've glued together a bunch of constraints. Obviously, these constraints are mutually exclusive; a number can't be equal to both 2 and 4 at the same time, so num == 2 and num == 4 would never evaluate to True. However, using or, only one of them needs to be True for the statement underneath to execute.

There's a little bit of intuition to it.

• "I want this AND this" has the implication of both at once.

• "I want this OR this" sounds more like either one would be adequate.

One other important tidbit, concerning not only conditionals, but also lists and booleans: the not keyword.

An often-important task in data science, when you have a list of things, is querying whether or not some new piece of information you just received is already in your list. You could certainly loop through the list, asking "is my new_item == list[item i]?". But, thankfully, there's a better way:

import random
list_of_numbers = [i for i in range(10)]  # Generates 10 random numbers, between 1 and 100.
if 13 not in list_of_numbers:
    print("Aw man, my lucky number isn't here!")

Aw man, my lucky number isn't here!

Notice a couple things here--

• List comprehensions make an appearance! Can you parse it out?

• The if statement asks if the number 13 is NOT found in list_of_numbers

• When that statement evaluates to True--meaning the number is NOT found--it prints the message.

If you omit the not keyword, then the question becomes: "is this number in the list?"

import random
list_of_numbers = [i for i in range(10)]  # Generates 10 random numbers, between 1 and 100.
if 13 in list_of_numbers:
    print("Somehow the number 13 is in a list generated by range(10)")

Nothing is printed in this case, since our conditional is asking if the number 13 was in the list. Which it's not.

Be careful with this. Typing issues can hit you full force here: if you ask:

if 0 in some_list

and it's a list of floats, then this operation will always evaluate to False.

Similarly, if you ask if "shannon" in name_list, it will look for the precise string "shannon" and return False even if the string "Shannon" is in the list. With great power, etc etc.

Part 2: Error Handling

Yes, errors: plaguing us since Windows 95 (but really, since well before then).

By now, I suspect you've likely seen your fair share of Python crashes.

• NotImplementedError from the homework assignments

• TypeError from trying to multiply an integer by a string

• KeyError from attempting to access a dictionary key that didn't exist

• IndexError from referencing a list beyond its actual length

or any number of other error messages. These are the standard way in which Python (and most other programming languages) handles error messages.

The error is known as an Exception. Some other terminology here includes:

• An exception is raised when such an error occurs. This is why you see the code snippet raise NotImplementedError in your homeworks. In other languages such as Java, an exception is "thrown" instead of "raised", but the meanings are equivalent.

• When you are writing code that could potentially raise an exception, you can also write code to catch the exception and handle it yourself. When an exception is caught, that means it is handled without crashing the program.

Here's a fairly classic example: divide by zero!

Let's say we're designing a simple calculator application that divides two numbers. We'll ask the user for two numbers, divide them, and return the quotient. Seems simple enough, right?

def divide(x, y):
    return x / y

divide(11, 0)

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-26-b8a694e48011> in <module>()
----> 1 divide(11, 0)

<ipython-input-18-0f39b0c6b46e> in divide(x, y)
      1 def divide(x, y):
----> 2     return x / y

ZeroDivisionError: division by zero

D'oh! The user fed us a 0 for the denominator and broke our calculator. Meanie-face.

So we know there's a possibility of the user entering a 0. This could be malicious or simply by accident. Since it's only one value that could crash our app, we could in principle have an if statement that checks if the denominator is 0. That would be simple and perfectly valid.

But for the sake of this lecture, let's assume we want to try and catch the ZeroDivisionError ourselves and handle it gracefully.

To do this, we use something called a try / except block, which is very similar in its structure to if / elif / else blocks.

First, put the code that could potentially crash your program inside a try statement. Under that, have a except statement that defines

1. A variable for the error you're catching, and
2. Any code that dictates how you want to handle the error

def divide_safe(x, y):
    quotient = 0
    try:
        quotient = x / y
    except ZeroDivisionError:
        print("You tried to divide by zero. Why would you do that?!")
    return quotient

Now if our user tries to be snarky again--

divide_safe(11, 0)

You tried to divide by zero. Why would you do that?!

0

No error, no crash! Just a "helpful" error message.

Like conditionals, you can also create multiple except statements to handle multiple different possible exceptions:

import random  # For generating random exceptions.
num = random.randint(0, 1)
try:
    # code for something can cause multiple exceptions
    pass
except NameError:
    print("Caught a NameError!")
except ValueError:
    print("Nope, it was actually a ValueError.")

Also like conditionals, you can handle multiple errors simultaneously. If, like in the previous example, your code can raise multiple exceptions, but you want to handle them all the same way, you can stack them all in one except statement:

import random  # For generating random exceptions.
num = random.randint(0, 1)
try:
    if num == 1:
        raise NameError("This happens when you use a variable you haven't defined")
    else:
        raise ValueError("This happens when you try to multiply a string")
except (NameError, ValueError):  # MUST have the parentheses!
    print("Caught...well, some kinda error, not sure which.")

Caught...well, some kinda error, not sure which.

If you're like me, and you're writing code that you know could raise one of several errors, but are too lazy to look up specifically what errors are possible, you can create a "catch-all" by just not specifying anything:

import random  # For generating random exceptions.
num = random.randint(0, 1)
try:
    if num == 1:
        raise NameError("This happens when you use a variable you haven't defined")
    else:
        raise ValueError("This happens when you try to multiply a string")
except:
    print("I caught something!")

I caught something!

Finally--and this is really getting into what's known as control flow (quite literally: "controlling the flow" of your program)--you can tack an else statement onto the very end of your exception-handling block to add some final code to the handler.

Why? This is code that is only executed if NO exception occurs. Let's go back to our random number example: instead of raising one of two possible exceptions, we'll raise an exception only if we flip a 1.

import random  # For generating random exceptions.
num = random.randint(0, 1)
try:
    if num == 1:
        raise NameError("This happens when you use a variable you haven't defined")
except:
    print("I caught something!")
else:
    print("HOORAY! Lucky coin flip!")

HOORAY! Lucky coin flip!

Review Questions

Some questions to discuss and consider:

1: Go back to the if / elif / else example about student grades. Let's assume, instead of elif for the different conditions, you used a bunch of if statements, e.g. if grade >= 90, if grade >= 80, if grade >= 70, and so on; effectively, you didn't use elif at all, but just used if. What would the final output be in this case?

2: We saw that you can add an else statement to the end of an exception handling block, which will run code in the event that no exception is raised. Why is this useful? Why not add the code you want to run in the try block itself?

3: With respect to error handling, we discussed try, except, and else statements. There is actually one more: finally, which executes no matter what, regardless of whether an exception occurs or not. Why would this be useful?

4: There's a whole field of "Boolean Algebra" that is not to different from the "regular" algebra you're familiar with. In this sense, rather than floating-point numbers, variables are either True or False, but everything still works pretty much the same. Take the following equation: a(a + b). Let's say a = True and b = False. If multiplication is the and operator, and addition is the or operator, what's the final result?

5: How would you write the following constraint as a Python conditional: $10 \lt x \le 20$, and $x$ is even.

Course Administrivia

• How was the review session last Friday?

• Assignment 2 is due tonight at 11:59:59pm. Let me know if there are any questions!

• Assignment 3 comes out tomorrow!

Additional Resources

1. Matthes, Eric. Python Crash Course. 2016. ISBN-13: 978-1593276034
2. Grus, Joel. Data Science from Scratch. 2015. ISBN-13: 978-1491901427