Exception swallowing in `finally`

The gotcha

A return, break, or continue inside finally silently suppresses the active exception. The exception you thought would propagate is gone.

Minimal repro

def f():
    try:
        raise ValueError("boom")
    finally:
        return 42      # swallows the ValueError

print(f())   # 42 — no exception raised

Same with break/continue in a loop:

for i in range(3):
    try:
        raise ValueError("boom")
    finally:
        break          # swallows the exception, exits the loop
print("survived")

Why it happens

Python’s spec: if finally clause executes a return/break/continue, that flow control supersedes any pending exception or in-progress return. The exception is just dropped.

This is rarely intentional. It’s almost always a bug — someone wrote finally: return last_value to “always return something” and accidentally swallowed errors.

How to avoid

Don’t put return in finally. Set state in the try and return after the block, or wrap differently.
If you genuinely want to convert exceptions to return values, do it explicitly with except:

def f():
    try:
        return compute()
    except ValueError:
        return 42      # explicit fallback
    # finally only does cleanup, no return

Use contextlib.suppress for “ignore this error” patterns, which is at least visible:

from contextlib import suppress
with suppress(FileNotFoundError):
    os.remove("maybe.txt")

Linters: pylint flags return-in-finally (W0150) and flake8-bugbear has B012.

Bonus: exceptions raised in `finally`

If the body raises and finally raises a different exception, the original is “chained” via __context__ but the new one propagates. Python 3 keeps the original visible in the traceback.

try:
    raise ValueError("first")
finally:
    raise RuntimeError("second")
# traceback shows: "during handling of the above ValueError, the following exception occurred"

Interview angle

“What does this print and what does it raise?” with a try/raise/finally/return snippet. The unwary answer “it raises ValueError” is wrong — the return in finally swallowed it.

Exception swallowing in finally