Conditional Statements

This lesson introduces conditional logic in Python, showing how to use if, elif, else, comparison operators, and logical operators to evaluate conditions.

Keywords

If statements, Elif else, Comparison operators, Boolean logic

Approximate time: XX minutes

Learning Objectives

In this lesson, we will:

• Create conditional statements using if, elif, and else
• Use the in operator to check for membership in a collection
• Use logical operators to test multiple conditions at the same time

Overview of lesson

Conditional statements are what let your code make decisions. In real projects, you might only run a quality‑control step if a file exists, flag samples that fail a cutoff, or choose a different analysis path depending on user input. All of these rely on if, elif, and else to check conditions and respond accordingly.

In this lesson, you will learn how to express True/False logic in Python so your scripts can adapt to different datasets and scenarios instead of doing the exact same thing every time.

if Statements

In Python, if is one of those key words that is off limits for variable names. It is used to create conditional statements, which allow us to execute certain blocks of code only if the specified conditions are met.

Essentially, we are evaluating whether or not a condition is True or False (boolean) and then executing code based on that result.

Syntax of if Statements

When we want to check a condition, we can use an if statement. Typically we are checking if a variable is:

• Greater than > or less than < a certain value
• Greater than or equal to >= or less than or equal to <= a certain value
• Equal to == a certain value
• Not equal to != a certain value

The basic syntax looks like this:

if condition:
    # code to execute if condition is true

So if we wanted to test x to see if it is greater than 10, we could write:

x = 15
if x > 10:
    print("x is greater than 10")

x is greater than 10

if statements are not exclusive to numeric comparisons, we can also check if a string is equal == to another string, if a list contains a certain value, and so on. So we can also write:

name = "Alice"
if name == "Alice":
    print("Hello, Alice!")

Hello, Alice!

Additionally, we are not limited to only one line of code in the code block. We can have as many lines of code as we want, as long as they are all indented at the same level.

name = "Alice"
if name == "Alice":
    print("Hello, Alice!")
    print("Welcome to the Python training!")

Hello, Alice!
Welcome to the Python training!

Indentation Matters

You may have noticed that the code block is indented. This is not just for readability, it is actually required in Python. The indentation tells Python which lines of code belong to the if statement. If you forget to indent, you will get an error.

x = 15
if x > 10:
print("x is greater than 10")

expected an indented block (<string>, line 3)

We can also have nested if statements. Notice that the level of indentation is increased for the inner if statement and will continue to increase for each additional level of nesting. For example:

x = 15
if x > 10:
    print("x is greater than 10")
    if x < 20:
        print("x is also less than 20")

x is greater than 10
x is also less than 20

in Operator

The in operator allows us to check if a value is present in a collection, such as a list or a string. For example, if we wanted to check if the letter “a” is in the string “hello”, we could write:

if "o" in "hello":
    print("The letter 'o' is in the string 'hello'")

The letter 'o' is in the string 'hello'

This can also be used to check if a value is in a list. We will discuss lists in more detail in the following lesson, but for now we can assess whether or not a value is in a list like so:

my_list = [1, 2, 3, 4, 5]
3 in my_list

True

As you saw, the in operator returns a boolean value (True or False) that indicates whether or not the value belongs to the list. This can be very useful for conditional statements, as we can use it to check if a value is in a list before trying to access it, which can help prevent errors. For example:

my_list = [1, 2, 3, 4, 5]
x = 3
if x in my_list:
    print(x, "is in the list!")

3 is in the list!

Warning

Have not written exercises for this lesson

Exercise 1

1. A question to evaluate Learning Objective 1
2. A followup question to question #1
3. …

Multiple Conditions with elif and else

We can join together the functionality of multiple if statements using elif and else. This allows us to check multiple conditions in a more efficient way without having to nest multiple if statements.

else Statements

Sometimes you may want to have a “catch-all” block of code that executes when the if statement is not met. This is where else comes in! We can build upon the “Alice” example from above and add an else statement to handle the case where name != "Alice":

name = "Will"
if name == "Alice":
    print("Hello, Alice!")
else:
    print("You aren't Alice!")

You aren't Alice!

elif Statements

Somewhat similarly to else, elif (short for “else if”) allows us to check multiple conditions in a sequence. The elif block will only be evaluated if the previous if or elif conditions were False. This allows us to check for multiple conditions without having to nest multiple if statements. For example:

name = "Bob"
if name == "Alice":
    print("Hello, Alice!")
elif name == "Bob":
    print("Hello, Bob!")
else:
    print("You aren't Alice or Bob!")

Hello, Bob!

You are not allowed to have an elif or else statement without a preceding if statement. However, you can have as many elif statements as you want in between the if and else statements. You are not required to include an else statement, but if you do, then it must be the last block in the sequence.

name = "Charlie"
if name == "Alice":
    print("Hello, Alice!")
else:
    print("You aren't Alice or Bob!")
elif name == "Bob":
    print("Hello, Bob!")

invalid syntax (<string>, line 6)

Order Matters

Another nuanced, but important point is that when we use if, elif, and else statements together, the order of the conditions matters. Python will evaluate the conditions in the order they are written, and once it finds a condition that is True, it will execute that block of code and skip the rest. For example:

x = 15
if x > 10:
    print("x is greater than 10")
elif x > 5:
    print("x is greater than 5 but not greater than 10")
else:
    print("x is 5 or less")

x is greater than 10

Exercise 2

1. A question to evaluate Learning Objective 2
2. A followup question to question #1
3. …

Logical Operators

Perhaps we wanted to test if two things were True at the same time, or if at least one of two things were True. This is where logical operators come in.

The and operator allows us to check if two conditions are both True, while the or operator allows us to check if at least one of two conditions is True. For example:

and Operator

Let us use the example from earlier where we were checking if x was greater than 10 and less than 20. We can use the and operator to check both conditions at the same time:

x = 15
if x > 10 and x < 20:
    print("x is greater than 10 and less than 20 ")
elif x > 5:
    print("x is greater than 5 but not greater than 10")
else:
    print("x is 5 or less")

x is greater than 10 and less than 20 

or Operator

If we wanted to check if x was either less than 5 or greater than 10, we could use the or operator:

x = 15
if x < 5 or x > 10:
    print("x is either less than 5 or greater than 10")
else:
    print("x is between 5 and 10")

x is either less than 5 or greater than 10

not Operator

The not operator allows us to check if a condition is False. For example, if we wanted to check if x is not greater than 10, we could write:

x = 15
if not x > 10:
    print("x is not greater than 10")
else:
    print("x is greater than 10")

x is greater than 10

This is sometimes more intuitive to read than writing if x <= 10:.

Combining Logical Operators

We can also combine logical operators to check multiple conditions at the same time. For example, if we wanted to check if x is greater than 10 and less than 20, or if x is equal to 5, we could write:

x = 15
if (x > 10 and x < 20) or x == 5:
    print("x is either greater than 10 and less than 20, or x is equal to 5")
else:
    print("x does not meet either condition")

x is either greater than 10 and less than 20, or x is equal to 5

Exercise 3

1. A question to evaluate Learning Objective 3
2. A followup question to question #1
3. …

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