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FDs will be closed after exception automatically in python2.7?


D'Arcy Cain <darcy at VybeNetworks.com> writes:
> On 2019-06-10 15:46, Alan Bawden wrote:
> > D'Arcy Cain <darcy at VybeNetworks.com> writes:
> >> with open("file","w+") as fd:
> > 
> > That makes the window smaller, but it doesn't actually eliminate it.  Look
> > at the generated byte code.  In both cases the call to open() is over and
> > the open file is created _before_ the SETUP_WITH instruction is executed.
> 
> Am I correct in assuming that the window is there for process interrupts
> but not threads?

I believe that _either_ a signal handler invocation _or_ a thread switch
might happen immediately before the SETUP_WITH instruction, so there is a
window in either case.

You probably do have less to worry about in the thread switch case,
because eventually the thread will resume where it left off (there being no
way to kill a thread).

In the interrupt case it is possible that a KeyboardInterrupt will kill
your thread right before the SETUP_WITH, and then the stream's __exit__
method will never be called.  But you don't have much to worry about in
that case either because the ONLY reference to the stream is on the top of
the stack, so after the thread is unwound the stream will become garbage,
and then the garbage collector will close it.

But my point was that replacing:

    f = open(...)
    with f:

with

    with open(...) as f:

doesn't fully close _any_ timing windows, it's just clearer to write it
that way.

-- 
Alan Bawden