Python Fundamentals: Write Your First Programs

Variables, Data Types, and Basic Operations

What Are Variables?

A variable is a named container that stores a value in your computer's memory. Think of it as a labeled box where you can put data and retrieve it later by using the name. Variables allow you to write flexible, reusable code by storing information that can change.

To create a variable in Python, you use the assignment operator (=):

name = "Alice"
age = 25
height = 5.7

Variable names should be descriptive and follow these rules: they must start with a letter or underscore, can contain letters, numbers, and underscores, and cannot be Python keywords (like if, for, or return). Python uses snake_case convention: user_age instead of userAge.

Understanding Data Types

Every value in Python has a data type that determines what kind of data it holds and what operations you can perform on it. The four fundamental data types are:

Integers (int): Whole numbers without decimals

count = 42
temperature = -5

Floats (float): Numbers with decimal points

price = 19.99
pi = 3.14159

Strings (str): Text enclosed in quotes (single or double)

greeting = "Hello, World!"
filename = 'document.txt'

Booleans (bool): True or False values used for logic

is_raining = True
game_over = False

You can check a variable's type using the type() function:

print(type(age))  # Output: <class 'int'>

Basic Operations

Arithmetic operations work with numbers (int and float):

• Addition: 10 + 5 = 15
• Subtraction: 10 - 5 = 5
• Multiplication: 10 * 5 = 50
• Division: 10 / 5 = 2.0 (always returns float)
• Floor division: 10 // 3 = 3 (discards remainder)
• Modulus: 10 % 3 = 1 (remainder only)
• Exponentiation: 2 ** 3 = 8

String operations work with text:

first = "John"
last = "Doe"
full_name = first + " " + last  # Concatenation
greeting = "Hello " * 3  # "Hello Hello Hello "

Comparison operations return boolean values:

5 > 3   # True
5 == 5  # True (equality)
5 != 3  # True (not equal)

Type Conversion

Sometimes you need to convert one data type to another using type conversion functions:

age_string = "25"
age_number = int(age_string)  # Convert string to integer

score = 95
score_text = str(score)  # Convert integer to string

value = "3.14"
decimal = float(value)  # Convert string to float

Practice Example

Let's combine these concepts:

name = "Emma"
hours_worked = 8
hourly_rate = 15.50

total_pay = hours_worked * hourly_rate
print(f"{name} earned ${total_pay} today")

Understanding variables, data types, and operations is essential because every program relies on storing and manipulating data. Master these foundations, and you'll be able to write more complex programs with confidence.

Getting User Input and Printing Output

Introduction

One of the most fundamental skills in programming is the ability to communicate with users. Every useful program needs to display information and receive data from people. In Python, we accomplish this with two essential functions: print() for output and input() for accepting user data. Mastering these functions will allow you to create interactive programs that respond to user needs.

The print() Function

The print() function displays text and data to the console (your screen). This is how your program communicates results back to the user.

Basic syntax:

print("Hello, World!")

You can print multiple items by separating them with commas:

print("My name is", "Alice", "and I am", 25)

The print() function automatically adds a newline at the end, moving the cursor to the next line. You can print numbers, strings, variables, and expressions:

age = 30
print("Next year I will be", age + 1)

One useful feature is the sep parameter, which controls what separates multiple items:

print("apple", "banana", "cherry", sep="-")  # Output: apple-banana-cherry

The input() Function

The input() function pauses your program and waits for the user to type something, then presses Enter. Whatever the user types is returned as a string.

Basic syntax:

name = input("What is your name? ")
print("Hello,", name)

Notice that input() takes a prompt string as an argument—this message tells the user what to type.

A critical concept: input() always returns a string, even if the user types numbers:

age_text = input("How old are you? ")
print(type(age_text))  # Output: <class 'str'>

Converting Input to Numbers

Since input() returns a string, you must convert it if you need numeric operations. Use int() for whole numbers and float() for decimals:

age = int(input("How old are you? "))
height = float(input("What is your height in meters? "))
print("In 10 years, you will be", age + 10)

If the user enters invalid data (like typing "abc" when you expect a number), your program will crash with an error. For now, assume users enter valid input—we'll handle errors in later lessons.

Putting It Together

Here's a complete interactive program:

print("Welcome to the age calculator!")
name = input("Enter your name: ")
birth_year = int(input("What year were you born? "))
current_year = 2024
age = current_year - birth_year
print(name, "you are approximately", age, "years old!")

This program demonstrates the flow: display instructions → receive input → process data → display results.

Summary

• Use print() to display output to the user
• Use input() to receive data from the user
• Remember that input() returns a string—convert to int() or float() for math
• Combine these functions to create interactive programs that respond to user needs

Practice writing programs that ask for user information and display personalized responses. This foundation is essential for all programming you'll do.

Conditionals: Making Decisions in Code

Imagine writing a program that simply executes the same steps in the same order every time. Real programs need to be smarter—they need to make decisions based on conditions. Conditionals are control structures that allow your program to take different actions depending on whether a condition is true or false. This is how programs respond intelligently to user input and changing circumstances.

Understanding the if Statement

The foundation of decision-making in Python is the if statement. Here's how it works:

if <expression>:
    <statement>

When Python encounters an if statement, it evaluates the expression (the condition). If the condition is truthy (evaluates to true), the statement or block of code directly below it executes. If the condition is falsy (evaluates to false), that code is skipped entirely.

For example:

age = 15
if age < 18:
    print("You are a minor")

Since age < 18 is true, the print statement runs. If age were 20, the condition would be false, and nothing would print.

The elif Statement

Often you'll want to check multiple conditions in sequence. This is where elif (else if) comes in. It allows you to test additional conditions if the first one is false:

age = 20
if age < 13:
    print("You are a child")
elif age < 18:
    print("You are a teenager")
elif age < 65:
    print("You are an adult")

Python evaluates these conditions from top to bottom and executes the first block where the condition is true, then skips the rest.

The else Statement

Finally, the else statement provides a fallback—code that runs if all previous conditions are false:

age = 70
if age < 18:
    print("You are a minor")
else:
    print("You are an adult")

You can combine all three together:

score = 75
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(f"Your grade is {grade}")

Blocks and Indentation

Notice that the code following a conditional statement is indented. Python uses indentation to define blocks of code that belong together. All statements at the same indentation level form a single block. This indentation is not optional—it's required syntax in Python.

Why Conditionals Matter

Conditionals transform your programs from rigid sequences of steps into flexible decision-makers. They enable your code to:

• Respond to user input ("If the user enters 'yes', do this")
• Handle different scenarios (different age groups, different score ranges)
• Validate data (Check if input is valid before processing)
• Control program flow (Skip unnecessary steps or repeat others)

By mastering if, elif, and else, you give your programs the ability to think—to choose different paths based on the situation at hand.