Chapter 3: Basic Syntax

Your First Python Program

Type python3 in your terminal to enter interactive mode, then type:

>>> print("Hello, world!")
>>> print("Hello, World!")

Hello, world!
Hello, World!

Congratulations! You just completed the "Hello World" ritual of programming 🎉

Variables — Naming Your Data

Variables are like labeled storage boxes — the label is the variable name, and what's inside the box is the data.

# Variable assignment
name = "Alice"           # String
age = 18                # Integer
height = 175.5          # Float
is_student = True       # Boolean

print(f"My name is {name}, I am {age} years old, and I am {height} cm tall")

My name is Alice, I am 18 years old, and I am 175.5 cm tall

📝 Note: 📝 Naming rules: Can only contain letters, numbers, and underscores _. Cannot start with a number (2name ❌, name2 ✅). Case-sensitive (Age and age are different variables). Cannot use Python keywords (like if, for, class). Recommended: lowercase with underscores (snake_case): my_name.

Data Types

# Basic data types in Python
text = "Hello"          # str - string
number = 42             # int - integer
price = 9.99            # float - floating point
is_open = True          # bool - boolean
nothing = None          # NoneType - null value

# Use type() to check type
print(type(text))
print(type(number))
print(type(price))
print(type(is_open))

<class 'str'>
<class 'int'>
<class 'float'>
<class 'bool'>

# Type conversion
age_str = "25"
age_int = int(age_str)      # String → Integer
age_float = float(age_str)  # String → Float
age_back = str(age_int)     # Integer → String

print(f"Type conversion example: {age_int} → {age_float} → {age_back}")

Type conversion example: 25 → 25.0 → 25

Operators

# Arithmetic operators
print(f"Addition: 10 + 3 = {10 + 3}")
print(f"Subtraction: 10 - 3 = {10 - 3}")
print(f"Multiplication: 10 * 3 = {10 * 3}")
print(f"Division: 10 / 3 = {10 / 3}")
print(f"Floor division: 10 // 3 = {10 // 3}")
print(f"Modulo: 10 % 3 = {10 % 3}")
print(f"Exponentiation: 2 ** 10 = {2 ** 10}")

# Comparison operators
print(f"\n5 > 3  → {5 > 3}")
print(f"5 == 5 → {5 == 5}")
print(f"5 != 3 → {5 != 3}")

# Logical operators
print(f"\nTrue and False → {True and False}")
print(f"True or False  → {True or False}")
print(f"not True       → {not True}")

Addition: 10 + 3 = 13
Subtraction: 10 - 3 = 7
Multiplication: 10 * 3 = 30
Division: 10 / 3 = 3.3333333333333335
Floor division: 10 // 3 = 3
Modulo: 10 % 3 = 1
Exponentiation: 2 ** 10 = 1024

5 > 3  → True
5 == 5 → True
5 != 3 → True

True and False → False
True or False  → True
not True       → False

Input and Output

# Output: print() function
print("Hello", "World", sep="-", end="!\n")

# Input: input() function (note: return value is always a string)
name = input("Enter your name: ")
age = int(input("Enter your age: "))  # Remember to convert type
print(f"Hello, {name}! Next year you'll be {age + 1} years old.")

Hello-World!
Enter your name: Bob
Enter your age: 20
Hello, Bob! Next year you'll be 21 years old.

💡 Tip: 💡 f-string magic: f-strings introduced in Python 3.6+ are the best way to format strings. Just add f before a string, and you can embed any expression using {}.