observer_pattern.py

class Publisher:
    
    def __init__(self):
        self.observers = []

    def add(self, observer):
        if observer not in self.observers:
            self.observers.append(observer)
        else:
            print(f"Failed to add: {observer}")
    
    def remove(self, observer):
        try:
            self.observers.remove(observer)
        except ValueError:
            print(f"Failed to remove {observer}")
    
    def notify(self):
        [o.notify(self) for o in self.observers]
    
class DefaultFormatter(Publisher):
    
    def __init__(self, name):
        Publisher.__init__(self)
        self.name = name
        self._data = 0
    
    def __str__(self):
        return f"{type(self).__name__}: '{self.name}' has data = {self._data}"
    
    @property
    def data(self):
        return self._data
    
    @data.setter
    def data(self, new_value):
        try:
            self._data = int(new_value)
        except ValueError as e:
            print(f"Error: {e}")
        else:
            self.notify()

class HexFormatterObs:
    def notify(self, publisher):
        value = hex(publisher.data)
        print(f"{type(self).__name__}: 'publisher.name' has now bin data = {value} ")

class BinaryFormatterObs:  
    def notify(self, publisher):  
        value = bin(publisher.data)
        print(f"{type(self).__name__}: '{publisher.name}' has now bin data = {value}")  
 
def main():
    df = DefaultFormatter("test1")
    print(df)

    print()
    hf = HexFormatterObs()
    df.add(hf)
    df.data = 3
    print(df)

    print()
    bf = BinaryFormatterObs()
    df.add(bf)
    df.data = 21
    print(df)

    print()
    df.remove(hf)
    df.data = 40
    print(df)

    print()
    df.remove(hf)
    df.add(bf)

    df.data = "hello"
    print(df)

    print()
    df.data = 15.8
    print(df)

if __name__ == "__main__":
    main()