An observer that can never leave is a memory leak. The clean idiom: subscribe hands back an unsubscribe function, so the caller can cancel without needing a reference to the subject or the list.
subscribe(observer): register the observer and return a function that removes it; calling it more than once must be safeclass Subject:
def __init__(self):
self._observers = []
def subscribe(self, observer):
"""Register observer and return a function that unsubscribes it."""
# TODO: add the observer to the end of self._observers, then build a closure
# over that observer which drops it from the list only while it is still
# present, so a repeat call is a harmless no-op. That closure is what the
# caller gets back.
pass
def notify(self, *args):
for observer in self._observers:
observer(*args)
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An observer that can never leave is a memory leak. The clean idiom: subscribe hands back an unsubscribe function, so the caller can cancel without needing a reference to the subject or the list.
subscribe(observer): register the observer and return a function that removes it; calling it more than once must be safePress Run Tests or ⌘↩ to check your solution.