[Python-ideas] adding an __exec__ method to context managers?

Sturla Molden sturla at molden.no
Tue Oct 13 17:31:51 CEST 2009

Sturla Molden skrev:
> I am not talking about OpenMP support, I am talking about how we use 
> Python threads. 

Which is to say that Java threads, which Python's threading module 
mimics, is a bad concurrency abstraction. It is error prone and 
difficult to use  (does not fit the programmer's mind).

Another ting is that some of this is to some extent a compensation for 
lack of os.fork in Windows. On Linux, one could call os.fork in 
__enter__,  and achieve much the same effect as an __exec__ method. That 
is, the parent forks, raises an exception (jumps to __exit__), and calls 
os.waitpid there. The child executes the "with ctxmgr:" block, and calls 
sys.exit (or os._exit) in __exit__.

That kind of scheme would not work on Windows (well there is Cygwin and 
SUA...) It would also be somewhat limited by the child and parent not 
sharing memory space. So I still think there is justification for a 
__exec__ method in context managers.

By the way: I tried to implement this using a bytecode hack a while ago. 
That is, spawn a thread and execute the code object from the calling 
stack frame in __enter__. It failed whenever the with block contained a 
loop, as there was some variable (I think it was called _1) that the 
interpreter could not find. And bytecode hacks are not particularly 
reliable and portable either.


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