[Python-Dev] PEP 343 - next steps

[Nick Coghlan]

Nick Coghlan wrote:
> Either way, the code generation for the with statement could simply 
> emit BLOCK_ASYNC as the first opcode, then ALLOW_ASYNC as the opcode 
> immediate preceding SETUP_FINALLY. Correct behaviour in the face of 
> asynchronous events is then guaranteed, even for cases where the 
> resource is acquired in EXPR, or when __enter__ is written in Python.

Coming up out of the implementation weeds, this can be described as 
follows:

"""
Atomic resource acquisition

The resource acquisition line that starts the with statement is 
executed atomically. That is, the Python interpreter prevents 
asynchronous events and automatic context switches while the resource 
is being acquired. Context switches due to the current thread becoming 
blocked (e.g. by attempting to acquire a synchronisation lock) will 
still occur, avoiding deadlock.

Automatic context switches or asynchronous events that would have 
occurred during evaluation of the resource acquisition line are 
deferred so that they occur immediately prior to the execution of the 
contained suite. This ensures that the acquired resource is still 
properly released in the face of an asynchronous exception such as 
KeyboardInterrupt.

To avoid blocking exceptions for an extended period, 'expensive' 
calculations can be carried out on the line preceding the with 
statement, and stored in a local variable for use in the resource 
acquisition:

   tmp = some_expensive_calculation()
   with tmp as rsrc:
     # Perform operations with resource

Note that, in this case, the actual resource acquisition should occur 
in tmp.__enter__, in order to enjoy the benefits of the atomic 
resource acquisition.
"""

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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