Day 2: Variables and Data Types

What You'll Learn Today

• What variables are and how to use them
• Numeric types (integers and floating-point numbers)
• String basics and operations
• Boolean type (True/False)
• Type conversion

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What Are Variables?

Variables are like "labeled boxes" for storing data. You give a value a name so you can reference or modify it later.

flowchart LR
    subgraph Variable["Variable = Labeled Box"]
        A["age = 25"]
        B["name = 'Taro'"]
        C["price = 1980.5"]
    end

    A --> A1["25"]
    B --> B1["'Taro'"]
    C --> C1["1980.5"]

    style Variable fill:#3b82f6,color:#fff
    style A1 fill:#22c55e,color:#fff
    style B1 fill:#8b5cf6,color:#fff
    style C1 fill:#f59e0b,color:#fff

Creating and Assigning Variables

In Python, use = (assignment operator) to assign values to variables:

# Create variables and assign values
age = 25
name = "Taro"
price = 1980.5

# Display variable values
print(age)    # 25
print(name)   # Taro
print(price)  # 1980.5

Reassigning Variables

Variable values can be changed later:

counter = 1
print(counter)  # 1

counter = 2
print(counter)  # 2

counter = counter + 1
print(counter)  # 3

Variable Naming Rules

Rule	Valid Examples	Invalid Examples
Can use letters, numbers, underscores	user_name, count2	-
Cannot start with a number	name1	1name ❌
Cannot use reserved words	my_class	class ❌
Case-sensitive	Age and age are different	-

Recommended naming style (snake_case):

# Good examples
user_name = "Taro"
total_price = 1000
max_retry_count = 3

# Examples to avoid
userName = "Taro"  # camelCase (not preferred in Python)
x = "Taro"         # Meaningless name

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Numeric Types

Python has two main numeric types.

Integers (int)

Numbers without decimal points:

age = 25
year = 2024
negative = -100
big_number = 1_000_000  # Underscores for readability

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

Floating-Point Numbers (float)

Numbers with decimal points:

pi = 3.14159
temperature = -5.5
price = 1980.0

print(type(pi))  # <class 'float'>

Numeric Operations

a = 10
b = 3

print(a + b)   # 13    Addition
print(a - b)   # 7     Subtraction
print(a * b)   # 30    Multiplication
print(a / b)   # 3.333... Division (always float)
print(a // b)  # 3     Floor division
print(a % b)   # 1     Modulus (remainder)
print(a ** b)  # 1000  Exponentiation

Compound Assignment Operators

count = 10

count += 5   # Same as count = count + 5
print(count)  # 15

count -= 3   # Same as count = count - 3
print(count)  # 12

count *= 2   # Same as count = count * 2
print(count)  # 24

Operator	Meaning	Example
+=	Add and assign	x += 5
-=	Subtract and assign	x -= 3
*=	Multiply and assign	x *= 2
/=	Divide and assign	x /= 4
//=	Floor divide and assign	x //= 2
%=	Modulus and assign	x %= 3

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Strings (str)

Strings represent text data. Enclose them in single quotes ' or double quotes ".

Creating Strings

# Both work
name = 'Taro'
message = "Hello, World!"

# Multi-line strings
long_text = """This is
a multi-line
string"""

print(long_text)

String Concatenation

first_name = "Taro"
last_name = "Yamada"

# Concatenate with +
full_name = first_name + " " + last_name
print(full_name)  # Taro Yamada

# Repeat with *
line = "-" * 20
print(line)  # --------------------

f-strings (Formatted String Literals)

The most convenient way to format strings, available since Python 3.6:

name = "Taro"
age = 25

# Using f-strings (recommended)
message = f"My name is {name} and I am {age} years old."
print(message)  # My name is Taro and I am 25 years old.

# Calculations are possible
price = 1000
tax = 0.1
print(f"Total with tax: ${price * (1 + tax)}")  # Total with tax: $1100.0

# Format specifications
pi = 3.14159265
print(f"Pi: {pi:.2f}")  # Pi: 3.14

String Methods

text = "  Hello, Python!  "

# Case conversion
print(text.upper())      # "  HELLO, PYTHON!  "
print(text.lower())      # "  hello, python!  "

# Strip whitespace
print(text.strip())      # "Hello, Python!"

# Replace
print(text.replace("Python", "World"))  # "  Hello, World!  "

# Split
words = "apple,banana,cherry".split(",")
print(words)  # ['apple', 'banana', 'cherry']

# Length
print(len(text))  # 18

String Indexing and Slicing

flowchart TB
    subgraph Index["Indices for 'Python'"]
        direction LR
        P["P"] --> Y["y"] --> T["t"] --> H["h"] --> O["o"] --> N["n"]
    end

    subgraph Positive["Positive Index"]
        P0["0"] --> P1["1"] --> P2["2"] --> P3["3"] --> P4["4"] --> P5["5"]
    end

    subgraph Negative["Negative Index"]
        N6["-6"] --> N5["-5"] --> N4["-4"] --> N3["-3"] --> N2["-2"] --> N1["-1"]
    end

    style Index fill:#3b82f6,color:#fff

text = "Python"

# Get a single character by index
print(text[0])   # P (first character)
print(text[2])   # t
print(text[-1])  # n (last character)

# Get substrings with slicing
print(text[0:3])  # Pyt (0 to 2)
print(text[2:])   # thon (2 to end)
print(text[:3])   # Pyt (start to 2)
print(text[::2])  # Pto (every other character)

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Boolean Type (bool)

Boolean type has only two values: True and False.

is_active = True
is_admin = False

print(type(is_active))  # <class 'bool'>

Comparison Operators

Comparison operators return Boolean values:

a = 10
b = 5

print(a > b)   # True  (greater than)
print(a < b)   # False (less than)
print(a >= b)  # True  (greater than or equal)
print(a <= b)  # False (less than or equal)
print(a == b)  # False (equal)
print(a != b)  # True  (not equal)

Logical Operators

x = True
y = False

print(x and y)  # False (True only if both are True)
print(x or y)   # True (True if either is True)
print(not x)    # False (negation)

Operator	Meaning	Example
and	Logical AND	True and False → False
or	Logical OR	True or False → True
not	Negation	not True → False

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Type Conversion

You can convert data between different types.

# String → Integer
age_str = "25"
age = int(age_str)
print(age + 5)  # 30

# Integer → String
number = 42
text = str(number)
print("Number: " + text)  # Number: 42

# String → Float
price_str = "19.99"
price = float(price_str)
print(price * 2)  # 39.98

# Float → Integer (truncates decimal)
pi = 3.14159
print(int(pi))  # 3

flowchart LR
    subgraph Conversion["Type Conversion"]
        A["str()"] --> A1["Convert to string"]
        B["int()"] --> B1["Convert to integer"]
        C["float()"] --> C1["Convert to float"]
        D["bool()"] --> D1["Convert to boolean"]
    end

    style Conversion fill:#3b82f6,color:#fff

Checking Types with type()

print(type(42))        # <class 'int'>
print(type(3.14))      # <class 'float'>
print(type("Hello"))   # <class 'str'>
print(type(True))      # <class 'bool'>

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Getting User Input

Use the input() function to receive user input:

name = input("What is your name? ")
print(f"Hello, {name}!")

# Convert to number if needed
age_str = input("Enter your age: ")
age = int(age_str)
print(f"Next year you'll be {age + 1}!")

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The None Type

None represents the absence of a value:

result = None
print(result)       # None
print(type(result)) # <class 'NoneType'>

# Check if value is None
if result is None:
    print("No result yet")

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Summary

Data Type	Description	Examples
int	Integer	42, -5, 1_000
float	Floating-point number	3.14, -0.5, 1.0e8
str	String	"Hello", 'Python'
bool	Boolean	True, False
None	No value	None

Key Takeaways

1. Variables use = for assignment and can be changed anytime
2. Use meaningful variable names (snake_case recommended)
3. f-strings make it easy to embed variables in strings
4. Use type() to check data types
5. Type conversion lets you transform data between types

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Practice Exercises

Exercise 1: Basics

Create the following variables and check each type with type():

• Your age (integer)
• Your height (floating-point)
• Your name (string)
• Whether you like programming (boolean)

Exercise 2: Calculation

Write a program that asks the user for two numbers and displays their sum, difference, product, and quotient.

Challenge

Write a program that asks the user for their birth year and calculates their current age. Display the result using an f-string like "You are XX years old."

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References

• Python Docs - Numeric Types
• Python Docs - Strings
• Python Docs - f-strings

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Next Up: In Day 3, you'll learn about "Control Flow." Master conditionals and loops to control your program's execution!