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How do you check whether a file or directory exists before operating on it, using pathlib? How would you implement a context manager that temporarily changes the current working directory and restores it afterward? How do you implement contextlib.ExitStack to dynamically manage a VARIABLE number of context managers determined at runtime? How would you write a reusable context manager decorator using contextlib.contextmanager that ALSO works when applied directly to a function, like @contextmanager-decorated functions support? How do you correctly handle character encoding when reading text files that might not be UTF-8, and what error occurs if you guess wrong? How would you implement asynchronous file I/O using aiofiles alongside an async context manager, to avoid blocking the event loop during file operations? How would you write a context manager that atomically writes to a file, avoiding leaving a corrupted or partially-written file if an exception occurs mid-write?
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How would you implement a context manager for managing a lock (like threading.Lock) while ALSO adding a timeout, raising an error if the lock can't be acquired quickly enough?

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Answer

Implement __enter__ to attempt lock.acquire(timeout=N), raising a custom TimeoutError if it fails, and __exit__ to always release the lock — combining the standard locking pattern with custom timeout-handling logic that plain 'with lock:' doesn't provide out of the box.
import threading

class TimedLock:
    def __init__(self, lock, timeout):
        self.lock = lock
        self.timeout = timeout
    def __enter__(self):
        if not self.lock.acquire(timeout=self.timeout):
            raise TimeoutError("Could not acquire lock in time")
        return self.lock
    def __exit__(self, *args):
        self.lock.release()

lock = threading.Lock()
with TimedLock(lock, timeout=5):
    critical_section()
Real-world example Preventing a thread from hanging indefinitely waiting for a lock, instead failing fast with a clear timeout error after a reasonable wait.

Common follow-ups: Why does __exit__ need to accept *args (exc_type, exc_value, traceback) even if it doesn't use them?

Concurrency (asyncio/threading/multiprocessing)
All 15 File I/O & Context Managers questions