Conditional Constructs. If Statement

In this article, we'll get acquainted with conditional constructs in Python — one of the most important tools in programming. Without them, our programs would be like a train that can only move straight ahead, without the ability to choose a path at a fork.

Conditional constructs allow a program to "make decisions" and choose different actions depending on circumstances.

What is a conditional construct?

A conditional construct is a tool that allows different blocks of code to be executed depending on whether a certain condition is true (True) or false (False).

In Python, the main conditional construct is the if operator. It checks if a condition is true, and if it is — executes the corresponding block of code.

Basic conditional operator constructs

If operator syntax

The basic syntax of the if conditional construct in Python looks like this:

Python 3.13
# Simple example of the if operator
>>> age = 18

>>> if age >= 18:
...     print("You are an adult!")

>>> print("This line will always execute, regardless of the condition.")
You are an adult!
This line will always execute, regardless of the condition.

Note several important details:

A colon : is placed after the condition
The code block that should execute when the condition is true must have an indentation (usually 4 spaces or one tab)
Once the indentation ends, it marks the end of the if block

Let's change the age value and look at the result:

Python 3.13
>>> age = 16

>>> if age >= 18:
...     print("You are an adult!")

>>> print("This line will always execute, regardless of the condition.")
This line will always execute, regardless of the condition.

In this case, the condition age >= 18 is not true (it's false), so the code block inside if doesn't execute, and we only see the second line.

Using else

Often we need to execute one block of code if the condition is true, and another if the condition is false. For this, the if-else construct is used:

Python 3.13
>>> temperature = 15

>>> if temperature > 20:
...     print("It's warm today, you can wear a t-shirt!")
... else:
...     print("It's cool today, take a jacket!")
It's cool today, take a jacket!

Here, if the temperature is above 20, the first block will execute. Otherwise, the block after else executes. In our example, the temperature is 15, which is less than 20, so the code in the else block executes.

The elif construct — "else if"

What if we have more than two choice options? For example, we want different messages for different temperature ranges. For this, the elif construct is used (short for "else if"):

Python 3.13
>>> temperature = 25

>>> if temperature < 0:
...     print("Very cold! Wear a warm jacket and hat!")
... elif temperature < 10:
...     print("Cold. Dress warmly.")
... elif temperature < 20:
...     print("Cool. A light jacket wouldn't hurt.")
... elif temperature < 30:
...     print("Warm. T-shirt and shorts will do.")
... else:
...     print("Hot! Bring water with you!")
Warm. T-shirt and shorts will do.

In this example, Python checks each condition in order. As soon as it finds the first true condition, it executes the corresponding code block, and all subsequent conditions are skipped. If none of the conditions are true, the else block executes (if present).

Important: Python checks conditions from top to bottom and stops as soon as it finds the first true condition. The order of conditions matters!

Advanced features of conditional constructs

Nested conditions

Conditional constructs can be nested inside each other, creating more complex logic:

Python 3.13
>>> age = 25
>>> has_license = True

>>> if age >= 18:
...     print("You are an adult.")

>>>     if has_license:
...         print("You can drive a car.")
...     else:
...         print("To drive a car, you need to get a license.")
... else:
...     print("You are a minor and cannot drive a car.")
You are an adult.
You can drive a car.

However, don't overuse nested conditions — code with many levels of nesting becomes difficult to read. Often such logic can be rewritten using logical operators.

Logical operators in conditions

Logical operators are used to create complex conditions:

and — logical AND (condition is true if both expressions are true)
or — logical OR (condition is true if at least one expression is true)
not — logical NOT (inverts the value of the expression)

Let's rewrite our example with nested conditions:

Python 3.13
>>> age = 25
>>> has_license = True

>>> if age >= 18 and has_license:
...     print("You are an adult and can drive a car.")
... elif age >= 18 and not has_license:
...     print("You are an adult, but to drive a car you need to get a license.")
... else:
...     print("You are a minor and cannot drive a car.")
You are an adult and can drive a car.

Such code is more readable and easier to maintain.

Let's look at another example with the or operator:

Python 3.13
>>> day = "Saturday"

>>> if day == "Saturday" or day == "Sunday":
...     print("It's a weekend! You can relax. 😎")
... else:
...     print("It's a workday. Need to go to work. 💼")
It's a weekend! You can relax. 😎

Ternary operator

For simple conditions, you can use the ternary operator — a compact form of writing a condition:

Python 3.13
>>> age = 20

# Regular notation using if-else
>>> if age >= 18:
...     status = "an adult"
... else:
...     status = "a minor"

>>> print(f"Regular notation: You are {status}")
Regular notation: You are an adult

# The same logic, but using the ternary operator
>>> status = "an adult" if age >= 18 else "a minor"
>>> print(f"Ternary operator: You are {status}")
Ternary operator: You are an adult

The syntax of the ternary operator:

Python 3.13
value_if_true if condition else value_if_false

The ternary operator makes the code more compact, but don't abuse it — complex conditions are better written in the regular format for readability.

Practical examples of using conditions

Conditional constructs are widely used in everyday programming. Let's look at several practical examples:

Checking if a number is even

One of the simple and frequently used applications of conditions is determining if a number is even:

Python 3.13
>>> number = 42

>>> if number % 2 == 0:
...     print(f"{number} is an even number")
... else:
...     print(f"{number} is an odd number")
42 is an even number

Password verification

Conditions help implement data verification against certain criteria:

Python 3.13
>>> password = "password123"

>>> if len(password) < 8:
...     print("Password is too short!")
... elif password.isalpha():
...     print("Password must contain not only letters!")
... elif password.isdigit():
...     print("Password must contain not only digits!")
... else:
...     print("Password is suitable!")
Password is suitable!

Understanding check

Let's check how well you've understood the topic of conditional constructs:

What will the following code output?

Python 3.13
x = 5
if x > 10:
    print("A")
elif x > 5:
    print("B")
elif x == 5:
    print("C")
else:
    print("D")