Chapter 1: Data Types and Variables — Your First Conversation with Python¶

How to Run Python¶

There are two ways to run Python.

CLI (Interactive) Mode¶

Type python or py in the terminal to get the >>> prompt.

>>> print("Hello, World!")
Hello, World!
>>> 2 + 3
5

REPL (Read-Evaluate-Print Loop): repeats input → evaluate → output. You can check results immediately, but everything disappears when you quit.

Script Mode¶

Create a .py file and run it.

hello.py → python hello.py

Since it's saved as a file, it's suitable for reuse, distribution, and automation.

Mode	Feature	Use Case
CLI (Interactive)	Immediate execution, lost on exit	Quick testing
Script	Saved as file, repeatable	Real programming

What Are Data Types?¶

Computers store everything as 0s and 1s.

01000001 = (int) 65  or  (str) 'A'

The same bits can mean different things depending on the type.

Data types tell Python two things: 1. What kind of value it is (a number? a string?...) 2. What operations are available (can you add it? concatenate it?...)

Python's Major Data Types¶

str         String        "hello", "abc"
int         Integer       42, -7, 0
float       Float         3.14, -0.5
bool        Boolean       True, False
list        List          [1, 2, 3]
tuple       Tuple         (1, 2, 3)
dict        Dictionary    {"key": "value"}

C vs Python — Dynamic Typing¶

// C: You must declare the type explicitly
int A = 42;

# Python: Just assign the value (dynamic typing)
A = 42

Python automatically determines the type based on the value.

Checking Types: `type()`¶

print(type(-3))       # <class 'int'>
print(type(3.0))      # <class 'float'>
print(type("314"))    # <class 'str'>
print(type(4 / 2))    # <class 'float'>  ← Note!

Value	Type
`-3`	int
`3.0`	float
`3.14`	float
`"314"`	str
`""`	str
`4/2`	float

str vs int Operations¶

# int: arithmetic
print(10 + 3)    # 13
print(10 * 3)    # 30

# str: concatenation, repetition
print("10" + "3")    # 103  ← string concatenation, not math!
print("10" * 3)      # 101010

Variables¶

What Is a Variable?¶

A variable is a name attached to a value.

A = 3

A: variable name
=: assignment operator
3: value — stored somewhere in memory

A = 3
print(A)    # 3
A = 99      # reassignment (rebinding)
print(A)    # 99

Checking Memory Address: `id()`¶

A = 3
print(id(A))      # e.g. 140234567890
print(hex(id(A))) # e.g. 0x7f8b2c3d4e50  (hexadecimal)

id() returns the memory address where the value is stored.

Variable Naming Rules¶

Valid names:

Name	Explanation
`_a`	Starts with underscore
`abcd`	Lowercase
`a3`	Letters + digits
`none`	Lowercase is not a keyword
`_1_`	Underscore and digits combined

Invalid names:

Name	Reason
`ab cd`	No spaces allowed
`3a`	Cannot start with a digit
`None`	Python reserved word
`True`	Python reserved word
`if`, `for`	Python reserved words
`a3.14`	No special characters

Rule summary: - Must start with a letter or _ - Case-sensitive (age ≠ Age) - No spaces, special characters, or Python reserved words

Built-in Functions¶

`print()` — Output¶

print("Hello, World!")     # Hello, World!
name = "Jimin"
print(name)                # Jimin
print("Name:", name)       # Name: Jimin

Thinking of it as a function:

f(x) = 2x
print("Hello") → input: "Hello" → output: Hello on screen

`input()` — Getting Input¶

name = input("Enter your name: ")
print("Hello,", name)

Enter your name: Jimin
Hello, Jimin

input() always returns a string (str).

age = input("Age: ")
print(type(age))     # <class 'str'>

# To use it as a number, convert the type
age = int(input("Age: "))
print(age + 1)

`type()` — Check Type¶

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

`id()` — Memory Address¶

x = 10
print(id(x))      # integer (memory address)
print(hex(id(x))) # displayed in hexadecimal

Practice Missions¶

Mission 1: Introduce Yourself¶

Create a program that asks for your name, age, and favorite subject using input(), then prints them out.

Enter your name: Jimin
Enter your age: 17
Favorite subject: Math
---
Hello! I'm Jimin.
Age: 17
Favorite subject: Math

Mission 2: Type Detective¶

Check the type of each value below using type() and compare your predictions with the actual results.

values = [-3, 3.0, 3.14, 314, "314", "", "int64", 4/2, True]
for v in values:
    print(v, "→", type(v))

Mission 3: Circle Area¶

Take the radius as input and calculate the area of a circle.

Area = π × r²   (π = 3.14)

r = float(input("Radius: "))
# Write your code here

# Radius: 5
# Circle area: 78.5

Mission 4: Variable Swap¶

Swap the values of a and b. (Use a temporary variable or Python's tuple unpacking)

a = 10
b = 20
# Write your code here
print(a)    # 20
print(b)    # 10

Key Summary¶

Concept	Description
CLI mode	`>>>` prompt, interactive execution
Script mode	Save and run `.py` files
Data type	Kind of value (int, float, str, bool...)
Dynamic typing	Python automatically determines types
Variable	A name attached to a value stored in memory
`print()`	Output to screen
`input()`	Receives user input as str
`type()`	Check data type
`id()`	Check memory address