Decorator in Python
Decorator is a structural pattern that allows adding new behaviors to objects dynamically by placing them inside special wrapper objects, called decorators.
Using decorators you can wrap objects countless number of times since both target objects and decorators follow the same interface. The resulting object will get a stacking behavior of all wrappers.
Complexity:
Popularity:
Usage examples: The Decorator is pretty standard in Python code, especially in code related to streams.
Identification: Decorator can be recognized by creation methods or constructors that accept objects of the same class or interface as a current class.
Conceptual Example
This example illustrates the structure of the Decorator design pattern. It focuses on answering these questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
main.py: Conceptual example
class Component():
"""
The base Component interface defines operations that can be altered by
decorators.
"""
def operation(self) -> str:
pass
class ConcreteComponent(Component):
"""
Concrete Components provide default implementations of the operations. There
might be several variations of these classes.
"""
def operation(self) -> str:
return "ConcreteComponent"
class Decorator(Component):
"""
The base Decorator class follows the same interface as the other components.
The primary purpose of this class is to define the wrapping interface for
all concrete decorators. The default implementation of the wrapping code
might include a field for storing a wrapped component and the means to
initialize it.
"""
_component: Component = None
def __init__(self, component: Component) -> None:
self._component = component
@property
def component(self) -> Component:
"""
The Decorator delegates all work to the wrapped component.
"""
return self._component
def operation(self) -> str:
return self._component.operation()
class ConcreteDecoratorA(Decorator):
"""
Concrete Decorators call the wrapped object and alter its result in some
way.
"""
def operation(self) -> str:
"""
Decorators may call parent implementation of the operation, instead of
calling the wrapped object directly. This approach simplifies extension
of decorator classes.
"""
return f"ConcreteDecoratorA({self.component.operation()})"
class ConcreteDecoratorB(Decorator):
"""
Decorators can execute their behavior either before or after the call to a
wrapped object.
"""
def operation(self) -> str:
return f"ConcreteDecoratorB({self.component.operation()})"
def client_code(component: Component) -> None:
"""
The client code works with all objects using the Component interface. This
way it can stay independent of the concrete classes of components it works
with.
"""
# ...
print(f"RESULT: {component.operation()}", end="")
# ...
if __name__ == "__main__":
# This way the client code can support both simple components...
simple = ConcreteComponent()
print("Client: I've got a simple component:")
client_code(simple)
print("\n")
# ...as well as decorated ones.
#
# Note how decorators can wrap not only simple components but the other
# decorators as well.
decorator1 = ConcreteDecoratorA(simple)
decorator2 = ConcreteDecoratorB(decorator1)
print("Client: Now I've got a decorated component:")
client_code(decorator2)
Output.txt: Execution result
Client: I've got a simple component:
RESULT: ConcreteComponent
Client: Now I've got a decorated component:
RESULT: ConcreteDecoratorB(ConcreteDecoratorA(ConcreteComponent))
