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Session 6 Part 2 Quiz (Semaphore Basics)

Question 1

What is a semaphore useful for?

  • Making a function recursive
  • Sorting strings alphabetically
  • Removing the need for error handling
  • Allowing up to a fixed number of workers into a block

Answer: 4 Type: single Time: 45 Explanation: A semaphore controls access by maintaining a limited number of permits.

Question 2

If a semaphore is created with threading.Semaphore(4), how many threads can enter the protected block at once?

  • 1
  • 2
  • 4
  • 40

Answer: 3 Type: single Time: 35 Explanation: The initial semaphore value gives 4 available permits.

Question 3

What is a permit in a semaphore?

  • A Python file extension
  • One available slot that lets a worker enter
  • A failed network request
  • A random sentence

Answer: 2 Type: single Time: 40 Explanation: A permit represents one available entry into the semaphore-controlled section.

Question 4

In the simple semaphore example, why can only two workers enter at once?

  • The semaphore is created with 2 permits
  • There are only two Python files
  • Threads always run in pairs
  • time.sleep(1) blocks all output forever

Answer: 1 Type: single Time: 45 Explanation: threading.Semaphore(2) allows two workers into the protected block at a time.

Question 5

Which statement best describes the difference between a mutex and a semaphore?

  • A mutex is only for HTTP requests
  • A semaphore always allows unlimited workers
  • They are unrelated to shared resources
  • A mutex allows one worker; a semaphore can allow a fixed number of workers

Answer: 4 Type: single Time: 50 Explanation: A mutex is one-at-a-time access, while a semaphore can allow N-at-a-time access.

Question 6

In the request exercise, why do we use a semaphore instead of starting all 40 calls without a limit?

  • To make every request fail
  • To respect a concurrency limit and avoid overwhelming a service
  • To remove the need for URLs
  • To stop Python from importing modules

Answer: 2 Type: single Time: 45 Explanation: The semaphore keeps only a controlled number of requests active at once.

Question 7

Why might the request exercise still need a separate write lock?

  • Locks cannot be used with requests
  • Files are always downloaded automatically
  • The semaphore limits requests, but file writing is a different shared resource
  • A write lock increases network speed

Answer: 3 Type: single Time: 50 Explanation: Limiting active requests and protecting shared file writes are separate coordination concerns.

Question 8

If 40 one-second tasks run with a limit of 4, how many waves of work do you expect approximately?

  • 1
  • 4
  • 10
  • 40

Answer: 3 Type: single Time: 45 Explanation: 40 tasks divided into groups of 4 gives about 10 waves, ignoring overhead and network variation.

Question 9

What should happen if one HTTP request fails?

  • Record the failure and continue
  • Hide the error silently
  • Stop writing all results
  • Delete the semaphore

Answer: 1 Type: single Time: 40 Explanation: A robust parallel task records failures instead of crashing the whole run.

Question 10

Which line creates a semaphore with four permits?

  • threading.Lock(4)
  • requests.Semaphore(4)
  • ThreadPoolExecutor.Semaphore(4)
  • threading.Semaphore(4)

Answer: 4 Type: single Time: 40 Explanation: threading.Semaphore(4) creates a semaphore with four available permits.