Conditionals

Introduction

So far, we have seen how to retrieve data from our code, and manipulate that data. What we have not learned, however, is how to make decisions with that data. Making decisions is something that we do every day in the real world. For example, if a restaurant is too expensive we may want to choose a different one. If it's too cold outside, we should find something to do inside. These are the types of decisions we want our code to make as well. After learning about conditionals we can do just that.

Learning Objectives

Understand how an if statement can change the execution flow of our code when certain conditions are met
Understand how the if keyword works with the else keyword in Python
See how to select certain data by combining if statements in for loops

If Statement and Execution Flow

So far in Python, all of our lines of code run one after the other. So in the code below, vacation_days is initially assigned to 0, then it is reassigned by incrementing by one, and again reassigned by incrementing again by one, which brings the vacation_days to a total of 2.

vacation_days = 0
vacation_days += 1
vacation_days += 1
vacation_days

The += is used to increment. The statement vacation_days += 1 can be thought of as vacation_days = vacation_days + 1. On line 2, vacation_days is 0. Then we reassign vacation_days to equal the previous value of vacation_days, which is 0, plus 1. Again we increment vacation_days on line 3, which would now equate to 1 + 1, and finally we output the new value of vacation_days, 2.

Contrast this with code that contains an if statement. Code that is part of an if block runs only when the conditional argument following the if evaluates to True. So it is not necessarily the case that every line of code runs.

vacation_days = 1
if False:
    # code does not run as conditional argument False
    vacation_days += 1

vacation_days

Above we can see that since the condition following the if equals False, the code directly underneath is not run. So, vacation_days stays assigned to the number 1.

Just as we did with functions, we indicate that something is part of the block by indenting. So the line vacation_days += 1 is indented to ensure that whether it is run depends on the conditional argument above. To end the block we simply stop indenting.

vacation_days = 1
if False:
    # if block begins
    vacation_days += 1
# if block ends
vacation_days += 2
vacation_days

So in the above cell, the last two lines are run because they are not part of the if block.

And, as you may have guessed, when the conditional argument is True, the code in the conditional block does run.

vacation_days = 1
if True:
    # code in if block runs, as True
    vacation_days += 1
vacation_days

Code that sometimes runs

Our code in conditional arguments becomes more interesting when we use conditional arguments that are less direct.

def long_vacation(number_of_days):
    if number_of_days > 4:
        return 'that is a long vacation'

long_vacation(5) # 'that is a long vacation'

'that is a long vacation'

long_vacation(3) # None

In the code above, you can hopefully see the power of our if statement. Our if argument is the expression number_of_days > 4, which sometimes evaluates to True and sometimes False, it depends on the number of days.

Now sometimes we want to say that when something is True do one thing, and when not True do something else.

def vacation_length(number_of_days):
    if number_of_days > 4:
        return 'that is a long vacation'
    else:
        return 'not so long'

vacation_length(3) # 'not so long'

'not so long'

vacation_length(5) # 'that is a long vacation'

'that is a long vacation'

Truthiness

So far our conditionals have depended on whether something evaluates exactly to True or False. But conditionals don't force us to be so precise. Conditionals also consider some values True if they are truthy and False if they are falsy. Take a look at the following:

vacation_days = 1
if vacation_days:
    # this is run
    vacation_days += 1
vacation_days

Even through vacation_days did not equal True above, it still ran the code in the if block because the value for vacation_days was 1, which is considered truthy.

However, 0 is not considered truthy.

vacation_days = 0
if vacation_days:
    # this is not run
    vacation_days += 1
vacation_days

Since 0 is not truthy, it is considered falsy. We can see that the if block was not run and vacation_days was not incremented, almost as if vacation_days evaluated to False.

So what is truthy and what is falsy in Python? Zero is falsy, and None is falsy. Also falsy is anything where len of that thing returns False, so '', [] are both falsy. Let's see that.

greeting = ''
if greeting:
    greeting += 'Hello'
else:
    greeting += 'Goodbye'
greeting

'Goodbye'

If we are ever curious about the whether something is truthy or falsy in Python, we can just ask with the bool function.

bool(0) # False

False

bool(1) # True

True

Conditionals in Loops

Finally, we can use conditionals in loops. This is great at filtering out certain elements and selecting just what we need. Let's see this.

greetings = ['hello', 'bonjour', 'hola', 'hallo', 'ciao', 'ola', 'namaste', 'salam']

def starts_with_h(words):
    selected = []
    for word in words:
        if word.startswith('h'):
            selected.append(word)
    return selected

starts_with_h(greetings)

['hello', 'hola', 'hallo']

The above starts_with_h function uses a for loop to move through the list of words one by one. For each word, it checks if the word starts with h and if it does, it adds that word to the selected list. Finally, the function returns that list of selected elements. So by using the for loop combined with if we can choose elements of a list based on a specific criteria.

Summary

In this lesson, we saw how conditionals allow us to make decisions with our code by only executing code under the if statement when the conditional argument is True or truthy. We then saw how we can use the else statement to only run code when the conditional argument is False or falsy, and as we know, code that is not in a conditional block is still run as normal.

We examined what is truthy or falsy, and saw that None, 0, and data with a length of zero are falsy. If we are unsure, we can use the bool function to see a the boolean value of a piece of data. Finally, we saw how by using if in a for loop we can return a subset of a collection that meets a criteria.

jmarkowi/conditionals-python-readme-data-science-intro-000