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classmethod() Decorator Complexity

The classmethod() decorator wraps a function so that it receives the class as its first argument instead of an instance. Applying it builds one small descriptor when the class body runs; each later access that reaches it, through the class or an instance, binds the function to the class.

d is the number of classes in the class's MRO, len(cls.__mro__). The bounds exclude the method body, which costs what it does.

Complexity Reference

Operation	Time	Space	Notes
classmethod(func), @classmethod	O(1)	O(1)	Once, when the class body runs
Cls.method, obj.method	O(1)	O(1)	Builds a new bound method on each access, with the class as __self__
Cls.method(...)	O(1)	O(1)	Plus the method body; the class is passed as the first argument
Looking up an inherited classmethod	O(1)	O(1)	The type attribute cache answers repeated lookups at any depth
First lookup after the class or a base changes	O(d)	O(d)	Walks the MRO, then the result is cached again
Lookup on a class modified and read over and over (3.13+)	O(d)	O(1)	The class stops being cached; see Class State and the Attribute Cache

Basic Usage

Define a Class Method

class Counter:
    created = 0

    @classmethod
    def create(cls):
        cls.created += 1  # O(1) - modifies the class
        return cls()

first = Counter.create()  # O(1)
second = Counter.create()
assert Counter.created == 2
assert isinstance(first, Counter)

Alternative Constructors

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    @classmethod
    def from_tuple(cls, point_tuple):
        x, y = point_tuple
        return cls(x, y)  # O(1)

    @classmethod
    def from_string(cls, point_str):
        x, y = map(float, point_str.split(","))  # O(len(point_str))
        return cls(x, y)

p1 = Point.from_tuple((1, 2))
p2 = Point.from_string("3.5,4.5")
assert (p1.x, p1.y) == (1, 2)
assert (p2.x, p2.y) == (3.5, 4.5)

Binding the Class

The class stores the classmethod object itself. Reading the attribute calls its __get__(), which builds a bound method whose __self__ is the class - also when the attribute is read through an instance, whose only contribution is its type. An instance attribute of the same name shadows the classmethod, because it is a non-data descriptor.

class Example:
    @classmethod
    def info(cls):
        return f"I am {cls.__name__}"

descriptor = Example.__dict__["info"]
assert type(descriptor) is classmethod

bound = Example.info  # O(1) - a new bound method
assert bound.__self__ is Example
assert Example.info is not Example.info  # a fresh object on each access
assert Example.info == Example.info  # but equal ones

obj = Example()
assert obj.info.__self__ is Example  # the instance is not passed
assert obj.info() == Example.info() == "I am Example"

Inheritance and the MRO

An inherited classmethod receives the class it was looked up on, so a factory written once builds instances of every subclass. Finding the method on a deep subclass walks the MRO only when the type attribute cache misses; repeated lookups cost the same at any depth.

class Base:
    @classmethod
    def create(cls):
        return cls()

class Child(Base):
    pass

obj = Child.create()  # O(d) the first time, then O(1) while cached
assert type(obj) is Child

# A staticmethod receives no class, so this factory names one
class HardcodedBase:
    @staticmethod
    def create():
        return HardcodedBase()

class HardcodedChild(HardcodedBase):
    pass

assert type(HardcodedChild.create()) is HardcodedBase

Class State and the Attribute Cache

Assigning a class attribute - cls.created += 1 in a classmethod - modifies the class, so the next lookup of an attribute found through the MRO of it or its subclasses misses the cache and walks the MRO. On Python 3.13+ a class that is modified and then read over and over - a counter bumped by every call - stops being cached at all, and from then on every lookup on it is O(d). Keep state that changes on every call in a mutable object the class holds: changing that object does not modify the class.

import itertools

class Order:
    _ids = itertools.count(1)  # the counter changes, the class does not

    def __init__(self, order_id):
        self.order_id = order_id

    @classmethod
    def create(cls):
        return cls(next(cls._ids))  # O(1) - no class attribute is assigned

first = Order.create()
second = Order.create()
assert (first.order_id, second.order_id) == (1, 2)

classmethod vs staticmethod

Both are found by the same attribute lookup. A staticmethod returns the plain function, so reading it builds nothing; a classmethod binds the class on every access.

class Both:
    @classmethod
    def from_class(cls):
        return cls

    @staticmethod
    def plain():
        return "no binding"

assert Both.plain is Both.plain  # O(1) - the function itself
assert Both.from_class is not Both.from_class  # O(1) - a new bound method each time
assert Both.from_class() is Both

Common Patterns

Plugin Registry

class Plugin:
    plugins = {}

    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        Plugin.plugins[cls.__name__] = cls  # O(1) - once per subclass definition

    @classmethod
    def create(cls, plugin_name):
        plugin_class = cls.plugins.get(plugin_name)  # O(1) average
        return plugin_class() if plugin_class else None

class AudioPlugin(Plugin):
    def play(self):
        return "Playing audio"

class VideoPlugin(Plugin):
    def play(self):
        return "Playing video"

assert Plugin.create("AudioPlugin").play() == "Playing audio"
assert Plugin.create("VideoPlugin").play() == "Playing video"
assert Plugin.create("Missing") is None

Conversion Constructors

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    @classmethod
    def from_list(cls, lst):
        return cls(lst[0], lst[1])  # O(1)

    @classmethod
    def from_dict(cls, d):
        return cls(d["x"], d["y"])  # O(1) average

    def to_list(self):
        return [self.x, self.y]

assert Vector.from_list([1, 2]).to_list() == [1, 2]
assert Vector.from_dict({"x": 3, "y": 4}).to_list() == [3, 4]

Performance Best Practices

✅ Do:

• Use a classmethod for alternative constructors, so subclasses get instances of themselves
• Keep per-call state, such as an ID counter, in a mutable object the class holds
• Use staticmethod when the function needs neither the class nor an instance - reading it builds no bound method

❌ Avoid:

• Assigning a class attribute on every call in a hot path - each assignment costs the class and its subclasses their cached lookups, and on 3.13+ can stop the class being cached at all
• Hardcoding the class name in a factory, which builds the base class for every subclass
• Wrapping property in classmethod - it computes a value on 3.11 and 3.12 only

Version Notes

• Python 3.13+: classmethod no longer wraps other descriptors: @classmethod over @property gives a bound method, not the property's value. The chaining was deprecated in 3.11
• Python 3.13+: A class modified and read over and over stops using the type attribute cache, and lookups on it are O(d)
• Python 3.10+: A classmethod copies __name__, __qualname__, __module__ and __doc__ from its function, and exposes the function as __wrapped__

Related Functions

• staticmethod() - A method with no implicit first argument, returned unbound
• property() - A computed attribute, the other common descriptor
• type() - The class a classmethod receives when called through an instance
• super() - Call the parent class's classmethod
• isinstance() - Check instance type