Objectives

By the end of this chapter, you should be able to:

• Write conditional logic using boolean operators
• List examples of falsey values in Python
• Use if/else to include conditional logic in your Python code
• Explain the difference between == and is in Python
• Convert between data types explicitly in Python

Boolean Logic

An essential part of writing programs is being able to execute code that depends on certain conditions. There are many different examples when you'd want to conditionally execute code. Here are just a few:

• You want the navigation bar on your website to look different based on whether or not someone is logged in
• If someone enters their password incorrectly, you want to let them know; otherwise, you want to log them in
• You're building a tic-tac-toe game, and want to know whether it's X's turn or O's turn
• You're building a social network and want to keep person A from seeing person B's profile unless the two of them are friends

And so on, and so on. It's very hard to write any kind of interesting software without making use of conditionals and boolean logic.

So let's talk about how to write conditional logic in Python. To do so, we'll make use of booleans (True and False), along with if statements.

Conditionals

Like many other programming languages, Python has conditionals with if/else statements. One difference with Python, however, is that it does not use the else if construct for chaining multiple conditions together. Instead, Python uses the keyword elif (short for else if). Note also that Python does not require parenthesis around conditions, but each condition must end with a :. If you forget the colon (a very common mistake when you're first learning Python!), you'll get a SyntaxError. Change the value of the user variable in the example below. Try to get each case to print to your console:

user = 'Elie'
if user == 'Elie':
    print('Awesome!')
elif user == 'Tom':
    print('Cool!')
else:
    print('Nope!')

Python also allows you to use words like or, and, and not for comparison

if 1 > 2 or 2 > 1:
    print("cool!")

if 1 == 1 and 2 == 2:
    print("nice!")

if not False:
    print("it is true!")

One nice thing about comparing numbers is that you can string inequalities together without using the and keyword:

if 1 < 2 and 2 < 3:
    print("this is ok")

if 1 < 2 < 3:
    print("this is better!")

Also, it's important to understand that the indentation in all of these examples matters tremendously. Indentation is how Python keeps track of what code should be executed conditionally, and which code should always be executed:

name = "Matt"
if name == "Matt":
    print("Your name is Matt")
print("Bye!")

In the example above, both messages will be printed. But if you change the name variable to something besides "Matt", the second print statement will still execute. Because of the indentation, Python knows that the last line is not part of the if statement!

If you forget about indentation, Python will throw an error. Python enforces indentation fairly strongly, but if you make a mistake, it'll let you know!

name = "Matt"
if name == "Matt":
print("Your name is Matt") # IndentationError!

Falsey Values in Python

Python has quite a few falsey values (values that evaluate to False when converted to a boolean). We can check whether a value is falsey by passing it into the bool function! All of the following examples evaluate to false when converted to a boolean.

# False
bool(False)

# 0
bool(0)

# None
bool(None)

# Empty string
bool("")

# Empty list
bool([])

# Empty tuple
bool(())

# Empty dictionary
bool({})

# Empty set
bool(set())

Comparison: is versus ==

Everything in Python is an object with an id. To see if two objects have the same id, you can use the is operator. You can also inspect an object's id directly using the id function.

• is operator (compares id)

a = 1 
b = a 
b is a # True
b = 2 
a is b # False
id(a) # should give you a number

If you just want to check whether two objects have the same value, you can use the == comparator. In general, it's probably best to use == for comparison, unless you know that you're trying to determine whether two objects have the same id (i.e. they are the same object in memory).

list1 = [1, 2]
list2 = [1, 2]
list3 = list1

list1 is list3 # True
list1 is list2 # False

list1 == list3 # True
list1 == list2 # True

Gathering user input

Before we move on to the exercises, there's one more function you should know about. It is very common in command line programs to prompt the user for information. In Python, this function is called input. Here's an example:

name = input("What is your name? ")
# you can now type anything and it will be saved into the name variable

pass

Since Python is very sensitive to indentation, there may be situations where you need to place indented code, but do not want to run anything. In this case, the pass keyword must be used.