The Tool Desk
Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Python’s @classmethod decorator makes the class itself the method’s implicit first argument, conventionally named cls. That lets a method use class-level information or create an instance of whichever class called it—including a subclass. Use an instance method when you need a particular object, a class method when you need the class, and a static method when neither implicit argument is needed.
What does @classmethod do?
The Python built-in functions documentation defines a class method as one that receives the class as an implicit first argument, just as an instance method receives an instance. By convention, that argument is named cls.
Decorate a function inside a class with @classmethod to make it a class method. You can call it through the class or through an instance. In either case, Python supplies the class—not the instance—as the first argument. If a derived class makes the call, Python supplies that derived class.
class User:
@classmethod
def guest(cls):
return cls("guest", is_active=True)
def __init__(self, name, is_active):
self.name = name
self.is_active = is_active
user = User.guest()Here, cls is User, so cls("guest", is_active=True) creates a User instance. Calling guest through an instance is also allowed, but that instance is not passed to the method; its class is.
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How to use a class method as an alternate constructor
An alternate constructor accepts a different representation of the data, converts or validates it, and returns a new instance. Use cls(...) when the constructor should create an instance of the class used for the call rather than always creating one fixed base class.
Example: construct from an ISO-style date string
class DateParts:
def __init__(self, year, month, day):
self.year = year
self.month = month
self.day = day
@classmethod
def from_iso(cls, text):
year, month, day = map(int, text.split("-"))
return cls(year, month, day)For example, calling DateParts.from_iso("2026-10-05") passes DateParts as cls, and the method returns an instance created from the parsed values. The example splits the string and converts each part to an integer; it does not perform comprehensive calendar validation.
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Why use cls instead of the base-class name?
If a subclass inherits from_iso and calls it, Python passes the subclass as cls. The constructor call therefore creates an instance of that subclass, provided its initializer accepts the supplied arguments. Hard-coding DateParts(...) would instead always construct the base class.
This dynamic binding is useful beyond custom constructors. The Python descriptor guide illustrates it with a Dict.fromkeys class method that creates and populates an instance using cls(), allowing the calling class to determine the result’s type.
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Class method vs. instance method vs. static method
| Method kind | Implicit first argument | Use it when |
|---|---|---|
| Instance method | The instance, conventionally self |
The operation needs or changes per-object state. |
| Class method | The class, conventionally cls |
The operation needs class-level information or should construct the class used for the call. |
| Static method | None | The function belongs conceptually in the class namespace but needs neither an instance nor a class argument. |
The descriptor guide describes class-method binding as supplying the object’s type when accessed through an object, or the class itself when accessed through a class. A static method passes through without adding an implicit argument. The practical distinction is that cls is dynamically bound, which is what makes subclass-aware construction possible; a static method has no such class argument.
Common classmethod mistakes
- Naming the first parameter
self. The parameter is the class object, not an instance. Use the conventional nameclsto make that clear. - Hard-coding the base class in a polymorphic constructor. If subclasses should get instances of their own type, return
cls(...)rather thanBaseClass(...). - Using a class method for a helper that needs no class behavior. A plain function or static method may communicate the intent more clearly when no implicit class argument is needed.
- Calling an instance method through the class without supplying an instance. An instance method expects its instance argument. Use a class method only when the operation genuinely belongs to the class rather than to one object.
Python version notes for class methods and descriptors
Python’s documented behavior has changed for class methods that wrap other descriptors, such as properties. The current built-in functions reference and descriptor guide record these relevant milestones:
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- Python 3.9: Class methods could wrap other descriptors.
- Python 3.10: Class methods began inheriting metadata attributes such as
__module__,__name__,__qualname__,__doc__, and__annotations__, and gained__wrapped__. - Python 3.11: Wrapping other descriptors with
classmethodwas deprecated. - Python 3.13: That descriptor-wrapping support was removed.
For current Python code, do not rely on a stacked @classmethod and @property pattern. The built-in reference documents classmethod as a descriptor that transforms a method into a class method; it is not a general-purpose way to make a class-level property.
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Choosing the right method type
- Choose an instance method if the behavior depends on the state of one object.
- Choose a class method if the behavior needs the class or should create an instance of the class through which it was called.
- Choose a static method if the function logically belongs in the class but needs neither an instance nor the class.
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