python-patterns/patterns/behavioral/observer.py at master · DataCraft-AI/python-patterns · GitHub

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"""
http://code.activestate.com/recipes/131499-observer-pattern/

*TL;DR
Maintains a list of dependents and notifies them of any state changes.

*Examples in Python ecosystem:
Django Signals: https://docs.djangoproject.com/en/3.1/topics/signals/
Flask Signals: https://flask.palletsprojects.com/en/1.1.x/signals/
"""

from __future__ importannotations

fromcontextlibimportsuppress
fromtypingimportList, Optional, Protocol


# define a generic observer type
classObserver(Protocol):
defupdate(self, subject: Subject) ->None:
pass


classSubject:
def__init__(self) ->None:
self._observers: List[Observer] = []

defattach(self, observer: Observer) ->None:
ifobservernotinself._observers:
self._observers.append(observer)

defdetach(self, observer: Observer) ->None:
withsuppress(ValueError):
self._observers.remove(observer)

defnotify(self, modifier: Optional[Observer] =None) ->None:
forobserverinself._observers:
ifmodifier!=observer:
observer.update(self)


classData(Subject):
def__init__(self, name: str="") ->None:
super().__init__()
self.name=name
self._data=0

@property
defdata(self) ->int:
returnself._data

@data.setter
defdata(self, value: int) ->None:
self._data=value
self.notify()


classHexViewer:
defupdate(self, subject: Data) ->None:
print(f"HexViewer: Subject {subject.name} has data 0x{subject.data:x}")


classDecimalViewer:
defupdate(self, subject: Data) ->None:
print(f"DecimalViewer: Subject {subject.name} has data {subject.data}")


defmain():
"""
 >>> data1 = Data('Data 1')
 >>> data2 = Data('Data 2')
 >>> view1 = DecimalViewer()
 >>> view2 = HexViewer()
 >>> data1.attach(view1)
 >>> data1.attach(view2)
 >>> data2.attach(view2)
 >>> data2.attach(view1)

 >>> data1.data = 10
 DecimalViewer: Subject Data 1 has data 10
 HexViewer: Subject Data 1 has data 0xa

 >>> data2.data = 15
 HexViewer: Subject Data 2 has data 0xf
 DecimalViewer: Subject Data 2 has data 15

 >>> data1.data = 3
 DecimalViewer: Subject Data 1 has data 3
 HexViewer: Subject Data 1 has data 0x3

 >>> data2.data = 5
 HexViewer: Subject Data 2 has data 0x5
 DecimalViewer: Subject Data 2 has data 5

 # Detach HexViewer from data1 and data2
 >>> data1.detach(view2)
 >>> data2.detach(view2)

 >>> data1.data = 10
 DecimalViewer: Subject Data 1 has data 10

 >>> data2.data = 15
 DecimalViewer: Subject Data 2 has data 15
 """


if__name__=="__main__":
importdoctest

doctest.testmod()