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How do you open and read a text file using the 'with' statement, and why is 'with' preferred over manual open()/close()? What is the difference between reading a file with .read(), .readline(), and .readlines()? What are the common file modes ('r', 'w', 'a', 'rb') passed to open(), and what does each one do? How does @contextlib.contextmanager let you write a context manager using a generator function instead of a full class? How do you use pathlib.Path for modern, object-oriented file path manipulation instead of os.path's string-based functions? How do you use multiple context managers in a single 'with' statement, and how does this compare to nesting separate 'with' blocks? 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?
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Python › File I/O & Context Managers

How do you write and use a custom context manager by implementing __enter__ and __exit__?

Intermediate

Answer

Define a class with __enter__ (running setup logic and returning the value bound by 'as') and __exit__ (running cleanup logic, receiving any exception info that occurred inside the 'with' block) — the 'with' statement automatically calls __enter__ on entry and __exit__ on exit, regardless of how the block ends.
class Timer:
    def __enter__(self):
        self.start = time.time()
        return self
    def __exit__(self, exc_type, exc_value, tb):
        elapsed = time.time() - self.start
        print(f"Took {elapsed:.2f}s")

with Timer():
    do_expensive_work()
Real-world example Building a custom resource manager (like a database connection or a performance timer) with guaranteed setup/teardown behavior.

Common follow-ups: What should __exit__ return if you want an exception raised inside the 'with' block to still propagate normally?

Exception Handling
All 15 File I/O & Context Managers questions