Class vs Instance vs Static Methods

Python offers three method flavors. Instance methods operate on objects, class methods on the class (often factories), and static methods as namespaced functions without implicit self or cls.

Recipe

class User:
    def __init__(self, email: str) -> None:
        self.email = email
 
    def domain(self) -> str:
        return self.email.split("@", 1)[1]
 
    @classmethod
    def from_row(cls, row: dict[str, str]) -> "User":
        return cls(row["email"])
 
    @staticmethod
    def is_valid_email(email: str) -> bool:
        return "@" in email and "." in email.split("@", 1)[-1]

When to reach for this:

• Alternative constructors (from_json, from_env)
• Utilities logically grouped with class but not needing state
• Inherited factory methods respecting subclass cls
• Enum/custom class methods modifying class state

Working Example

from datetime import datetime, timezone
 
class Event:
    def __init__(self, name: str, ts: datetime) -> None:
        self.name = name
        self.ts = ts
 
    def age_seconds(self, now: datetime | None = None) -> float:
        now = now or datetime.now(timezone.utc)
        return (now - self.ts).total_seconds()
 
    @classmethod
    def now(cls, name: str) -> "Event":
        return cls(name, datetime.now(timezone.utc))
 
    @classmethod
    def from_iso(cls, name: str, iso: str) -> "Event":
        ts = datetime.fromisoformat(iso.replace("Z", "+00:00"))
        return cls(name, ts)
 
    @staticmethod
    def normalize_name(name: str) -> str:
        return name.strip().lower()
 
class AdminEvent(Event):
    pass
 
if __name__ == "__main__":
    base = Event.now("deploy")
    admin = AdminEvent.now("promote")
    print(type(base), type(admin))
    print(Event.is_valid_email("a@b.com"))

What this demonstrates:

• now classmethod is alternative constructor
• AdminEvent.now returns AdminEvent because cls is subclass
• age_seconds instance method uses object state
• normalize_name staticmethod needs neither self nor cls

Deep Dive

How It Works

• Bound methods - Instance methods bind self at call time.
• classmethod descriptor - Passes class as first argument automatically.
• staticmethod descriptor - No automatic first argument.
• Inheritance - cls in classmethod is actual class called (AdminEvent not Event).
• Access - Classmethods can touch class attributes; instance methods instance attrs.

When to Use Which

Python Notes

# enum-style classmethod pattern
class Money:
    def __init__(self, amount: int, currency: str) -> None:
        self.amount = amount
        self.currency = currency
 
    @classmethod
    def usd(cls, amount: int) -> "Money":
        return cls(amount, "USD")

Gotchas

• staticmethod when classmethod needed - Factory returning cls breaks on subclasses with staticmethod. Fix: Use classmethod for polymorphic constructors.
• classmethod accessing instance state - No self available. Fix: Pass instance explicitly or use instance method.
• Confusing module-level function - staticmethod that never touches class - could be module function. Fix: staticmethod only when grouping API on class namespace.
• Abstract classmethod order - Stack @classmethod and @abstractmethod per Python version docs.
• Calling classmethod on instance - Allowed but unusual - still passes real class.

Alternatives

FAQs

Related

• Properties & Descriptors - method descriptors
• dataclasses - construction patterns
• Inheritance & MRO - cls in MRO context
• Enums - enum class methods

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

Y29kZWd1aWRlcy5pb3xjZ2lvNjc3fDIwMjYwNw==