[Python-ideas] Conditional context manager

[Neil Girdhar]

FYI: There is a null context manager: ExitStack().

On Sat, Oct 1, 2016 at 3:43 PM MRAB <python at mrabarnett.plus.com> wrote:

> On 2016-10-01 19:07, Neil Girdhar wrote:
> > I'm just throwing this idea out there to get feedback.
> >
> > Sometimes, I want to conditionally enter a context manager.  This
> > simplest (afaik) way of doing that is:
> >
> >     with ExitStack() as stack:
> >         if condition:
> >             cm = stack.enter_context(cm_function())
> >         suite()
> >
> > I suggest a more compact notation:
> >
> >     with cm_function() as cm if condition:
> >         suite()
> >
> > I'm not sure that this is possible within the grammar.  (For some reason
> > with with_expr contains '"as" expr' rather than '"as" NAME'?
> >
> > I realize this comes up somewhat rarely.  I use context managers a lot,
> > and it comes up maybe 1 in 5k lines of code.
> >
> > For some extensions of this notation, an else clause could bind a value
> > to cm in the case that condition is false.
> >
> If you defined a null context manager, you could then write:
>
>      with (cm_function() if condition else cm_null()) as cm:
>          suite()
>
> Do you need 'cm' itself? Its type changes depending on the condition, so
> I don't see how it could be useful.
>
> If it's not needed, then that shortens a little to:
>
>      with cm_function() if condition else cm_null():
>          suite()
>
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
>
> --
>
> ---
> You received this message because you are subscribed to a topic in the
> Google Groups "python-ideas" group.
> To unsubscribe from this topic, visit
> https://groups.google.com/d/topic/python-ideas/dcu3O1qaC3E/unsubscribe.
> To unsubscribe from this group and all its topics, send an email to
> python-ideas+unsubscribe at googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20161001/c0a8d168/attachment.html>