Python classmethod() Function: Usage and Examples

Use the classmethod() descriptor without decorator shorthand.

What You Will Learn

classmethod(function) converts a function in a class namespace to one receiving cls as the first argument when called through the class or an instance. @classmethod is syntactic shorthand.

Decorators are usually clearer unless attributes are being constructed dynamically.

Step-by-Step Example

The example below makes the behavior visible with small input values that you can change safely.

class Tracker:
    def build(cls, initial_count):
        instance = cls()
        instance.count = initial_count
        return instance

    build = classmethod(build)

tracker = Tracker.build(6)
print(tracker.count)

How the Example Works

The class Tracker groups data and behavior; its methods run when called on an instance.
The variable tracker holds the result of Tracker.build(6). Later lines use that value or inspect how it changes.
The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.

For the chosen input, the expression tracker.count prints 6. Compare that result with the rule above before you change the argument.

Expected output from the example:

When You Would Use This

These tools are useful when composing small reusable functions, adapting callbacks, or eliminating repeated work. Pay attention to whether the operation returns a new object, updates state, or produces lazy results. Test a small success case and a boundary case before integrating it into an application.

Practice: Modify the Code

Rewrite a classmethod declared with @classmethod using the built-in descriptor.

Verify the changed output against the behavior described above, including the relevant invalid or boundary case.

For complete parameters and version details, consult the official Python documentation.

Common Beginner Mistakes

Expecting a classmethod to receive self instead of the owning class.
Referring to a hardcoded base class instead of cls when constructing subclass instances.

Key Takeaways

Use the classmethod() descriptor without decorator shorthand.
classmethod(function) converts a function in a class namespace to one receiving cls as the first argument when called through the class or an instance. @classmethod is syntactic shorthand.
Decorators are usually clearer unless attributes are being constructed dynamically.