Variables and Data Types in Python

Variables are the foundation of any programming language. In Python, variables are like containers that store data values. Understanding how to work with different data types is crucial for writing effective Python programs.

What are Variables?

A variable is a named location in memory that stores a value. Think of it like a labeled box where you can put different items.

# Creating variables
name = "Alice"
age = 25
height = 5.6
is_student = True

Python Data Types

Python has several built-in data types:

1. Numeric Types

Integers (int)

score = 100
temperature = -5
population = 1000000

Floating-point numbers (float)

price = 19.99
pi = 3.14159
weight = 68.5

Complex numbers (complex)

z = 3 + 4j
complex_num = complex(2, 3)

2. Text Type

Strings (str)

message = "Hello, World!"
name = 'Python'
multiline = """This is a
multiline string"""

String operations:

# Concatenation
greeting = "Hello" + " " + "World"

# Repetition
pattern = "Python " * 3  # "Python Python Python "

# Length
length = len("Hello")  # 5

# Indexing and slicing
text = "Python"
first_char = text[0]  # 'P'
last_char = text[-1]  # 'n'
substring = text[1:4]  # 'yth'

3. Boolean Type

Boolean (bool)

is_active = True
is_finished = False

# Boolean operations
result = True and False  # False
result = True or False   # True
result = not True        # False

Variable Assignment

Simple Assignment

x = 10
name = "John"

Multiple Assignment

# Assign same value to multiple variables
a = b = c = 5

# Assign different values
x, y, z = 1, 2, 3
name, age = "Alice", 30

Swapping Variables

a = 10
b = 20
a, b = b, a  # Now a=20, b=10

Type Checking and Conversion

Checking Types

value = 42
print(type(value))  # <class 'int'>
print(isinstance(value, int))  # True

Type Conversion

# String to number
num_str = "123"
num = int(num_str)  # 123
float_num = float("123.45")  # 123.45

# Number to string
age = 25
age_str = str(age)  # "25"

# Boolean conversion
print(bool(1))    # True
print(bool(0))    # False
print(bool(""))   # False
print(bool("Hi")) # True

Variable Naming Rules

Valid Names

name = "Alice"
age2 = 25
_private = "secret"
CONSTANT = 100
firstName = "John"

Invalid Names

# These will cause errors:
# 2age = 25        # Can't start with number
# first-name = ""  # Can't use hyphens
# class = "Math"   # Can't use keywords

Naming Conventions

# Use descriptive names
user_count = 50  # Good
uc = 50         # Avoid abbreviations

# Constants in UPPERCASE
MAX_USERS = 1000
PI = 3.14159

# Functions and variables in snake_case
def calculate_area():
    pass

user_name = "Alice"

Common Operations

String Formatting

name = "Alice"
age = 30

# f-strings (Python 3.6+)
message = f"My name is {name} and I'm {age} years old"

# .format() method
message = "My name is {} and I'm {} years old".format(name, age)

# % formatting (older style)
message = "My name is %s and I'm %d years old" % (name, age)

Input from User

# Getting user input (always returns string)
name = input("Enter your name: ")
age_str = input("Enter your age: ")
age = int(age_str)  # Convert to integer

Practice Exercises

Try these exercises to reinforce your understanding:

Exercise 1: Personal Information

# Create variables for personal information
first_name = "Your name here"
last_name = "Your last name"
age = 0  # Your age
height_cm = 0  # Your height in cm
is_student = True  # True or False

# Print a formatted message
full_name = first_name + " " + last_name
print(f"Hello! I'm {full_name}, {age} years old, {height_cm}cm tall.")

Exercise 2: Calculator

# Simple calculator
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))

sum_result = num1 + num2
difference = num1 - num2
product = num1 * num2
quotient = num1 / num2

print(f"Sum: {sum_result}")
print(f"Difference: {difference}")
print(f"Product: {product}")
print(f"Quotient: {quotient}")

Exercise 3: Type Conversion Practice

# Convert between types
string_number = "42"
integer_number = int(string_number)
float_number = float(string_number)

print(f"String: {string_number} (type: {type(string_number)})")
print(f"Integer: {integer_number} (type: {type(integer_number)})")
print(f"Float: {float_number} (type: {type(float_number)})")

Common Mistakes to Avoid

1. Using reserved keywords as variable names
2. Forgetting to convert input strings to numbers
3. Mixing data types without proper conversion
4. Using unclear variable names

Summary

Understanding variables and data types is fundamental to Python programming. Remember:

• Variables are containers for storing data
• Python has several built-in data types
• Use descriptive variable names
• Convert between types when necessary
• Practice with real examples

In the next lesson, we'll explore control structures and learn how to make decisions in your Python programs!