We can create an empty class using pass statement in Python.
# An empty class
class Test:
pass
The question here is why we need empty classes?
Or do not know under what circumstances we are suppose to use an empty class.
I can't image the usage in daily development. Could anyone give a demonstration and explanation?
When you create a new class, you are creating a new type. This newly created type may have some properties, or may not. These properties allow to hold data, methods etc.
Empty class that doesn't inherit any base class would be useful as placeholder.
class ServiceWrapper(object):
pass
def sendMessage(svc:ServiceWrapper):
#do something
#pass
On the other hand, empty classes that inherit other classes is a very common pattern. Specially when defining user exceptions.
class Networkerror(RuntimeError):
pass
try:
raise Networkerror()
except Networkerror:
#do something
#pass
Also recently, the python collections.abc allows creating interface like functionalities.
class ServiceWrapper(ABC):
@abstractmethod
def send(self):...
def sendMessage(svc:ServiceWrapper):
svc.send()
#pass
I recently found myself using empty Python classes as unique "tags" for an observer system.
For example, something like this...
from collections import defaultdict
class OnCreate:
pass
class OnModify:
pass
class OnDelete:
pass
class ObserverSystem:
def __init__(self):
self.observers = defaultdict(list)
def register(self, event, callback):
self.observers[event].append(callback)
def notify(self, event, *args, **kwargs):
for callback in self.observers[event]:
callback(*args, **kwargs)
observer = ObserverSystem()
observer.register(OnCreate, lambda entity: print(f"Entity {entity} created"))
observer.register(OnModify, lambda entity: print(f"Entity {entity} modified"))
observer.register(OnDelete, lambda entity: print(f"Entity {entity} deleted"))
observer.notify(OnCreate, 1)
observer.notify(OnModify, 2)
observer.notify(OnDelete, 3)
I guess, I could have used a of numeric value, or even a string as the "tag", but the class is unique, it's hashable so I can use it as dict key, etc. Seems to work well.
