[Python-ideas] Conditional context manager

[Neil Girdhar]

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.

Best,

Neil
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