@classmethod
- Python course
- Intermediate
- Lesson 25/116
- 5 min
Learn class methods with @classmethod.
The @classmethod decorator
The @classmethod decorator takes cls as its first parameter. cls is the class itself, not an instance. This is useful for class-level operations.
Example: class Counter:\n @classmethod\n def from_list(cls, items):\n return cls(len(items)). In Counter.from_list([1, 2, 3]) cls is Counter; cls(len(items)) creates a new instance. This pattern is common as an alternate constructor.
What does cls represent?
cls is the class that calls the method. It gives access to the class, not instance data. Writing cls(...) creates a new instance; this makes alternate constructors easy.
Example: in Counter.from_list([1, 2, 3]) cls is Counter. With cls(len(items)) a Counter(3) instance is built. This also works in subclasses; cls is always the calling class.
Practice Add a classmethod to Counter
Define the Counter class. Add def __init__(self, count): with self.count = count in the body. Add @classmethod and def from_list(cls, items):; in the body write return cls(len(items)). Then create an instance with c = Counter.from_list([1, 2, 3]) and write print(c.count).
Starter code
# Define the Counter classSample solution
# Define the Counter class
class Counter:
def __init__(self, count):
self.count = count
@classmethod
def from_list(cls, items):
return cls(len(items))
c = Counter.from_list([1, 2, 3])
print(c.count)Expected output
3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the classmethod signature
class Counter:
@classmethod
def from_list(____, items):
return cls(len(items))class Counter:
@classmethod
def from_list(cls, items):
return cls(len(items))Explanation: @classmethod takes cls as its first parameter.
2 What is the first parameter of @classmethod?
- cls Correct answer
- self
- this
- class
Explanation: @classmethod takes cls as its first parameter.
3 Match the classmethod concepts
| Term | Meaning |
|---|---|
@classmethod |
Decorator |
cls |
Class reference |
cls(len(items)) |
New instance |