Given the following snapshot of the observer registry in a thread-safe observer pattern implementation: ```python observers = { 'eventA': {obs1, obs3}, 'eventB': {obs2} } ``` If a notify call for 'eventA' results in only obs3 receiving the update, which of the following is the most likely cause?

hard🔄 Reverse Engineer Q9 of Q15

OOP & Design Patterns - Observer Pattern - Event System, Publish-Subscribe

Aobs1 unsubscribed from 'eventA' but the registry was not updated

Bobs1's update method is overridden to ignore notifications

CThe notify method incorrectly filters observers by event type

Dobs3 is subscribed to multiple event types and receives all notifications

Step-by-Step Solution

Solution:

Step 1: Analyze observer registry
Both obs1 and obs3 are subscribed to 'eventA'.
Step 2: Consider notify behavior
Notify calls update on all subscribed observers; if only obs3 receives update, obs1 must be ignoring or suppressing updates.
Step 3: Eliminate other causes
Registry shows obs1 subscribed, so unsubscribe not done; notify filters correctly; obs3's multiple subscriptions irrelevant.
Final Answer:
Option B -> Option B
Quick Check:
Observer's update method can suppress notifications [OK]

Quick Trick: Subscribed observer may ignore updates internally [OK]

Common Mistakes:

MISTAKES

Assuming unsubscribe not done
Blaming notify filtering
Ignoring observer internal logic

Trap Explanation:

PITFALL

Candidates often assume registry state matches notification, ignoring observer internal behavior.

Interviewer Note:

CONTEXT

Tests deep understanding of observer notification flow and internal observer behavior.

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Solution Trace

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Given the following snapshot of the observer registry in a thread-safe observer pattern implementation: ```python observers = { 'eventA': {obs1, obs3}, 'eventB': {obs2} } ``` If a notify call for 'eventA' results in only obs3 receiving the update, which of the following is the most likely cause?

Step 1: Analyze observer registry

Step 2: Consider notify behavior

Step 3: Eliminate other causes

Final Answer:

Quick Check:

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