So far, we have seen how 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.

• 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 to combine if statements in for loops

So far in Python, all of our lines of code have run one after the other.

vacation_days = 0
vacation_days += 1
vacation_days

1

The += is used to increment. The statement vacation_days += 1 can be thought of as vacation_days = vacation_days + 1. So before line 2, vacation_days is 0. Then we reassign vacation_days to equal the previous value of vacation_days, 0, plus one.

Contrast this with code with an if statement. Once we run an if statement, this is no longer the case. Code that is part of an if block runs does not run when the conditional argument is False.

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

vacation_days

1

Because the code after 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 we indented the line vacation_days += 1 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

3

So above, the last two lines are run as 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

2

Our code in conditional arguments becomes more interesting if 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'
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 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'
vacation_length(5) # 'that is a long vacation'

'that is a long vacation'

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

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

2

However, 0 is not considered truthy.

vacation_days = 0
if vacation_days:
    # this is run
    vacation_days += 1
vacation_days

0

So because of that, the if block is not run, vacation_days is not incremented, and it stays at 0. Just as if vacation_days equaled False.

So what is truthy and what is falsy in Python? Zero is falsy, None is falsy. Also considered 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
bool(1) # True

True

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']

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. Finally, we saw how by using if in a for loop we can return a subset of a collection that meets a criteria.