Python ExitStack: unwind partially acquired resources

Operation contract

The import acquires two owned in-memory streams before a third acquisition fails. Previously acquired streams close through the stack even though the main import never starts. Register cleanup immediately after each successful acquisition. A separate close callback records order so the failure path can be inspected without using a real service.

Failure and ownership boundary

ExitStack.callback does not suppress an exception. Context managers entered through enter_context may have different exit behavior. A callback that raises can change the visible failure, so cleanup itself needs a failure policy. The stack does not close resources merely because it is garbage collected. Python context managers: clean up on success and failure and Python exceptions: translate an input error without hiding its cause supply the underlying rules.

Working program

from contextlib import ExitStack
from io import StringIO

closed = []
def release(stream, label):
    stream.close()
    closed.append(label)

streams = []
try:
    with ExitStack() as stack:
        for label in ("receipts", "prices"):
            stream = StringIO(label)
            streams.append(stream)
            stack.callback(release, stream, label)
        raise OSError("third input unavailable")
except OSError:
    print("acquisition failed")
print(closed)
print(all(stream.closed for stream in streams))

Output

acquisition failed
['prices', 'receipts']
True

Costs and limits

Registering k callbacks retains O(k) cleanup state. Unwinding takes the sum of their costs. Resource acquisition and close latency are external to the stack bookkeeping and may block.

Common Mistakes

• Register cleanup before attempting the next acquisition.
• Do not depend on stack garbage collection to release resources.

Connected lessons

Python context managers: clean up on success and failure, Python pathlib files: specify encoding and close the resource owner, Python exceptions: translate an input error without hiding its cause.

Follow the ownership and update boundary

Python ExitStack pop_all: transfer cleanup without discarding it.