Conditional operators in Python

Let's talk about conditional operators in Python.

What are Booleans?

We have three lists here, two of which represent the same data.

>>> a = [1, 2, 3]
>>> b = [1, 2, 3]
>>> c = [4, 5, 6]

If we wanted to ask whether two objects represent the same thing in Python, we can use the equals-equals (==) operator (a.k.a. the "equality operator"):

>>> a == b
True

The == operator will respond with either True or False.

>>> a == c
False

True and False are both of the type bool, which stands for Boolean:

>>> type(True)
<class 'bool'>

Booleans are how we represent "yes and no" or "affirmative and negative" in Python.

Booleans are often returned when using comparison operations, like equality (==).

Expressions that return either True or False (like a == b) are sometimes called Boolean expressions.

Where are Boolean expressions used?

Boolean expressions are often used in if statements to control whether a block of code should be run:

>>> a = [1, 2, 3]
>>> b = [1, 2, 3]
>>> if a == b:
...     print("The two lists are equal")
...
The two lists are equal

But they can also be used in other contexts, such as while loops.

Here's a greet.py program which uses a while loop:

name = ""
while name == "":
    # Loop until non-blank name is entered
    name = input("Please enter your name: ")

print(f"Hi {name}!")

We're looping as long as the user hasn't entered a name (as long as name is an empty string). Once they enter a name, we break out of our loop, and then we print out their name:

$ python3 greet.py
Please enter your name:
Please enter your name:
Please enter your name:
Please enter your name: Trey
Hi Trey!

Boolean expressions can also be stored in a variable.

Here, we're asking whether the length of our colors list is not equal to the length of the set of colors:

>>> colors = ["purple", "green", "blue", "purple", "yellow"]
>>> has_duplicates = len(colors) != len(set(colors))

It turns out, that's True (because we have a duplicate color and sets remove duplicates):

>>> has_duplicates
True

Examples of comparison operations

Booleans usually appear as the result of a comparison operation.

The equals-equals operator (==) is our equality operator in Python.

>>> n = -4
>>> n == 0
False

Python also has an inequality operator, != (sometimes called the not equals operator):

>>> n != 0
True

There's also less than and greater than:

>>> n < 0
True
>>> n > 0
False

And less than or equal to and greater than or equal to:

>>> n <= 0
True
>>> n >= 0
False

Python also has a containment operator, the in operator which checks whether something is contained within a collection (e.g. contained in a list):

>>> n = 5
>>> numbers = [2, 1, 3, 4, 7]
>>> n in numbers
False

And there's a not in operator for doing the opposite (checking for a lack of containment):

>>> n not in numbers
True

There's also the is operator and the is not operator for checking identity:

>>> n = 5
>>> n is None
False
>>> n is not None
True

You usually won't want to check for identity though. You'll typically want to check for equality instead of identity (see equality vs identity in Python).

Conditional operators return Boolean values

Python's conditional operators return Booleans (i.e. True and False values) which can be used to control the flow of your code with if statements and while loops.