[Python-Dev] PEP 377 - allow __enter__() methods to skip the statement body

Steven Bethard steven.bethard at gmail.com
Sun Mar 15 19:09:57 CET 2009

On Sun, Mar 15, 2009 at 10:50 AM, Michael Foord
<fuzzyman at voidspace.org.uk> wrote:
> Brett Cannon wrote:
>> Without knowing what StatementSkipped is (just some singleton? If so why
>> not just used SkipStatement instance that was raised?) and wondering if we
>> are just going to continue to adding control flow exceptions that directly
>> inherit from BaseException or some ControlFlowException base class, the
>> basic idea seems fine by me.
> Note that using exceptions for control flow can  be bad for other
> implementations of Python. For example exceptions on the .NET framework are
> very expensive. (Although there are workarounds such as not really raising
> the exception - but they're ugly).
> Isn't it better practise for exceptions to be used for exceptional
> circumstances rather than for control flow?

If my understanding is correct, the primary use case for this is when
an exception is raised by an __enter__() method and caught by an
enclosing __exit__() method. So at least in that case, you've already
incurred the cost of an exception.

It might be nice to see an example of this being used with only a
single context manager. Is that possible?

I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a
tiny blip on the distant coast of sanity.
        --- Bucky Katt, Get Fuzzy

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