Python
Observer
Learn the Observer pattern.
Observer
The Observer pattern lets an object notify its subscribers about state changes. The Subject keeps subscribers in a list; when notify is called, each gets the message.
class Subject:
def __init__(self):
self._observers = []
def subscribe(self, obs):
self._observers.append(obs)
def notify(self, value):
for obs in self._observers:
obs(value)
Subscribers are callable; this is common in event-driven interfaces and messaging systems.
Subscribing and notifying
To add a subscriber, call subscribe(obs); obs is callable (a function or an object with a __call__ method). Notification is done with notify(value).
def log_observer(value):
print(f"log: {value}")
s = Subject()
s.subscribe(log_observer)
s.notify("merhaba") # log: merhaba
Multiple subscribers can be added; all get the same value. This is good for logging, cache invalidation and UI updates.
Exercise Log with Observer
Define class Subject:. In __init__ set self._observers = []. Method def subscribe(self, obs): does self._observers.append(obs). Method def notify(self, value): does for obs in self._observers: obs(value). Function def log_observer(value): does print(f"log: {value}"). Create s = Subject(), call s.subscribe(log_observer) and s.notify("merhaba").
Show example solution
class Subject:
def __init__(self):
self._observers = []
def subscribe(self, obs):
self._observers.append(obs)
def notify(self, value):
for obs in self._observers:
obs(value)
def log_observer(value):
print(f"log: {value}")
s = Subject()
s.subscribe(log_observer)
s.notify("merhaba")
Test yourself
-
Where does the Subject keep subscribers in the Observer pattern?
- In a set
- In a file
- In a list
- In a database
Show answer
In a list — The Subject keeps subscribers in the _observers list.
-
Complete the notify body
def notify(self, value):
for obs in self._observers:
____(value)
Show answer
def notify(self, value):
for obs in self._observers:
obs(value)
-
Match the Observer concepts
- Subject
- Notifies
- subscribe
- Adds a subscriber
- notify
- Informs
- obs(value)
- Calls a subscriber
Next lesson: Decorator
