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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 do you write and use a custom context manager by implementing __enter__ and __exit__? 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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How does @contextlib.contextmanager let you write a context manager using a generator function instead of a full class?

Intermediate

Answer

Decorate a generator function with @contextmanager; code BEFORE the single 'yield' runs as __enter__ (the yielded value becomes the 'as' target), and code AFTER 'yield' (typically wrapped in try/finally) runs as __exit__ — a much more concise alternative to writing a full class with two dunder methods.
from contextlib import contextmanager
import time

@contextmanager
def timer():
    start = time.time()
    try:
        yield
    finally:
        print(f"Took {time.time() - start:.2f}s")

with timer():
    do_expensive_work()
Real-world example Writing a lightweight, function-based context manager without the boilerplate of a full class-based __enter__/__exit__ implementation.

Common follow-ups: Why must the yield inside a @contextmanager-decorated generator typically be wrapped in a try/finally?

Decorators
All 15 File I/O & Context Managers questions