All 15 File I/O & Context Managers questions
More in File I/O & Context Managers
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 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 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? 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?
Advertisement
Python › File I/O & Context Managers

How do you implement contextlib.ExitStack to dynamically manage a VARIABLE number of context managers determined at runtime?

Advanced

Answer

ExitStack lets you enter() any number of context managers PROGRAMMATICALLY (in a loop, or conditionally), and guarantees ALL of them are properly exited (in reverse order) when the ExitStack itself exits — essential when the exact number of resources to manage isn't known until runtime, unlike a fixed 'with a, b, c:' statement.
from contextlib import ExitStack

filenames = ["a.txt", "b.txt", "c.txt"]
with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in filenames]
    for f in files:
        process(f)
# ALL files are properly closed here, however many there were
Real-world example Opening and safely managing a dynamic, runtime-determined number of files or database connections in a single batch operation.

Common follow-ups: How does ExitStack's callback() method let you register arbitrary cleanup functions, not just full context managers?

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