For review: PEP 343: Anonymous Block Redux and Generator Enhancements

[Andrew Dalke]

Steven Bethard wrote:
> Ahh, so if I wanted the locking one I would write:
> 
>      with locking(mutex) as lock, opening(readfile) as input:
>          ...

That would make sense to me.

> There was another proposal that wrote this as:
> 
>      with locking(mutex), opening(readfile) as lock, input:
>          ...
> 
> which is what was confusing me.  Mirroring the 'as' from the import 
> statement seems reasonable.

Ahh, you're right.  That was an earlier proposal.

> 
> But it doesn't address my other concern, namely, is
> 
>      with locking(mutex), opening(readfile) as input:
>          ...
> 
> equivalent to the nested with-statements, e.g.:

I would think it's the same as

with locking(mutex):
  with opening(readfile) as input:
    ...

which appears to map to the first of your alternatives

> Or is it equivalent to something different, perhaps:
> 
>      _locking = locking(mutex)
>      _opening = opening(readfile)
>      _exc = (None, None, None)
>      _locking.__enter__()
>      input = _opening.__enter__()
>      try:
>          try:
>              ...
>          except:
>              _exc = sys.exc_info()
>              raise
>      finally:
>          _opening.__exit__(*exc)
>          _locking.__exit__(*exc)

That wouldn't work; consider if _opening.__enter__() raised
an exception.  The _locking.__exit__() would never be called,
which is not what anyone would expect from the intent of
this PEP.

> Or maybe:
> 
>      _locking = locking(mutex)
>      _opening = opening(readfile)
>      _exc = (None, None, None)
>      _locking.__enter__()
>      input = _opening.__enter__()

Same problem here

>      finally:
>          # same order as __enter__ calls this time!!
>          _locking.__exit__(*exc)
>          _opening.__exit__(*exc)

and the order would be wrong since consider multiple
statements as

with server.opening() as connection, connection.lock(column) as C:
  C.replace("X", "Y")

The inner with depends on the outer and must be closed
in inverted order.


> And if it *is* just equivalent to the nested with-statements, how often 
> will this actually be useful?  Is it a common occurrence to need 
> multiple with-statements?  Is the benefit of saving a level of 
> indentation going to outweigh the complexity added by complicating the 
> with-statement?

Agreed.

				Andrew
				dalke at dalkescientific.com