return in finally can swallow exception

[iritkatriel]

In the expr_match function:

def expr_match(ob, ed, c=InstanceDict, r=0):
    e, md, push, pop = ed
    push(c(ob, md))
    try:
        r = e.eval(md)
    finally:
        pop()
        return r

The return in the finally will swallow any in-flight exception. This means that if r.eval(md) raises any exception (including KeyboardInterrupt), this exception will be swallowed. This is probably not what the author intended.

See also: https://docs.python.org/3/tutorial/errors.html#defining-clean-up-actions.

It is possible that the intention was to write:

def expr_match(ob, ed, c=InstanceDict, r=0):
    e, md, push, pop = ed
    push(c(ob, md))
    try:
        r = e.eval(md)
    finally:
        pop()
    return r

which would return r is there is no exception, but allow the exception to propagate if there is one.

[davisagli]

I agree this should probably be corrected, but in the case of an exception in the try block, I think this raises UnboundLocalError since r hasn't been assigned.

[iritkatriel]

in the case of an exception in the try block, I think this raises UnboundLocalError since r hasn't been assigned.

In this case it wouldn't reach the 'return r', it will bubble the exception up.