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Consider the following Python code implementing a thread-safe Singleton with double-checked locking. What will be the output of the print statement?

easy🧾 Code Trace Q3 of Q15
OOP & Design Patterns - Singleton Pattern - Thread Safety, Double-Checked Locking & Lazy Init
Consider the following Python code implementing a thread-safe Singleton with double-checked locking. What will be the output of the print statement?
ATrue
BFalse
CRaises RuntimeError
DNone
Step-by-Step Solution
Solution:

  1. Step 1: Trace instance creation for s1

    _instance is None, lock acquired, instance created and assigned.

  2. Step 2: Trace instance creation for s2

    _instance is not None, returns existing instance.

  3. Final Answer:

    Option A -> Option A

  4. Quick Check:

    s1 and s2 refer to the same Singleton instance [OK]
Quick Trick: Double-checked locking returns same instance after first creation [OK]
Common Mistakes:
MISTAKES
  • Ignoring double-checked locking causes multiple instances
Trap Explanation:
PITFALL
  • Candidates may think second call creates new instance without checking _instance first.
Interviewer Note:
CONTEXT
  • Tests ability to trace thread-safe lazy singleton code
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