Observer
- Python course
- Advanced
- Lesson 64/83
- 5 min
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: merhabaMultiple subscribers can be added; all get the same value. This is good for logging, cache invalidation and UI updates.
Practice 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").
Starter code
# define Subject class (observers list, subscribe and notify)
# define log_observer function (prints value)
# create a Subject, subscribe log_observer and call notifySample 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")Expected output
log: merhaba
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Where does the Subject keep subscribers in the Observer pattern?
- In a set
- In a file
- In a list Correct answer
- In a database
Explanation: The Subject keeps subscribers in the _observers list.
2 Complete the notify body
def notify(self, value):
for obs in self._observers:
____(value) def notify(self, value):
for obs in self._observers:
obs(value)Explanation: obs(value) calls each subscriber.
3 Match the Observer concepts
| Term | Meaning |
|---|---|
Subject |
Notifies |
subscribe |
Adds a subscriber |
notify |
Informs |
obs(value) |
Calls a subscriber |