What's the difference between an instance method, a classmethod, and a staticmethod?

The three method types side by side

class Pizza:
    def __init__(self, toppings):
        self.toppings = toppings

    def add_topping(self, topping):        # instance method: needs a specific pizza
        self.toppings.append(topping)

    @classmethod
    def margherita(cls):                    # classmethod: alternative constructor
        return cls(["mozzarella", "tomato"])

    @staticmethod
    def is_valid_topping(name):             # staticmethod: no self/cls needed
        return name.lower() in KNOWN_TOPPINGS

• add_topping needs self because it mutates a specific pizza's toppings — calling it makes no sense without an instance.
• margherita is a factory method: Pizza.margherita() builds a new instance without the caller needing to know the constructor's exact argument list. cls refers to the actual class it's called on, so a subclass calling margherita() correctly builds that subclass, not hard-coded to Pizza.
• is_valid_topping doesn't touch instance or class state at all — it's grouped inside Pizza purely for namespacing/discoverability (Pizza.is_valid_topping("basil")), and could just as well be a module-level function.

Why classmethod respects subclassing and staticmethod doesn't apply

class StuffedCrustPizza(Pizza):
    pass

StuffedCrustPizza.margherita()   # returns a StuffedCrustPizza, not a Pizza!

Because margherita uses cls(...) rather than Pizza(...), calling it on a subclass constructs the subclass. This is the main reason to prefer @classmethod factories over hardcoding the class name.

When to choose which

Interview-ready summary: Instance methods operate on self; classmethods operate on cls and are the standard pattern for alternative constructors that respect subclassing; staticmethods are plain functions namespaced under a class with no implicit self/cls access at all.