class / __init__() / self

class ClassName:
    # Class variable (shared across all instances)
    class_var = value

    def __init__(self, arg):
        # Instance variable (unique to each instance)
        self.variable = arg

    def method(self):
        return self.variable

# Basic class definition
class Person:
    # Class variable (shared by all instances)
    species = 'Human'
    count = 0

    def __init__(self, name, age):
        # Instance variables
        self.name = name
        self.age  = age
        Person.count += 1  # Update the class variable

    def greet(self):
        return f"Hello! I'm {self.name}, age {self.age}."

    def birthday(self):
        self.age += 1
        return f"{self.name} is now {self.age} years old!"

    def __str__(self):
        return f"Person(name={self.name}, age={self.age})"

    def __repr__(self):
        return f"Person('{self.name}', {self.age})"

    def __del__(self):
        Person.count -= 1

# Creating instances
p1 = Person('Alice', 30)
p2 = Person('Bob', 25)

print(p1.greet())       # Hello! I'm Alice, age 30.
print(p2.birthday())    # Bob is now 26 years old!
print(Person.count)     # 2 (class variables are accessed via the class name)
print(p1.species)       # Human (also accessible from an instance)

# Checking __str__ and __repr__
print(str(p1))      # Person(name=Alice, age=30)
print(repr(p1))     # Person('Alice', 30)
print(p1)           # Person(name=Alice, age=30) (print() uses __str__)

# Difference between instance variables and class variables
p1.species = 'Earthling'   # Creates an instance variable on p1 (class variable is unchanged)
print(p1.species)           # Earthling
print(p2.species)           # Human (class variable is unchanged)
print(Person.species)       # Human (class variable is unchanged)

# __init__ with a default argument
class Circle:
    PI = 3.14159

    def __init__(self, radius=1.0):
        self.radius = radius

    def area(self):
        return Circle.PI * self.radius ** 2

c1 = Circle(5)
c2 = Circle()       # radius defaults to 1.0
print(c1.area())    # 78.53975
print(c2.area())    # 3.14159

Class variables are shared across all instances of a class. If you assign a value to a class variable through an instance, Python creates a new instance variable on that instance instead — the class variable itself remains unchanged. When using class variables for shared counters or configuration, always access them via the class name to avoid this pitfall.

Design __str__() to return a human-readable string, and __repr__() to return a detailed string for debugging. Ideally, __repr__() should return a string that eval() can use to recreate the object. When that is not practical, the convention is to use the format <ClassName attribute_info>.