[Python-Dev] PEP 310 and exceptions

Alex Martelli aleaxit at yahoo.com
Sat Apr 23 05:15:10 CEST 2005

On Apr 22, 2005, at 16:51, holger krekel wrote:

> Moreover, i think that there are more than the "transactional"
> use cases mentioned in the PEP.  For example, a handler
> may want to log exceptions to some tracing utility
> or it may want to swallow certain exceptions when
> its block does IO operations that are ok to fail.

I entirely agree!  In fact, I was discussing this very issue recently 
with colleagues at Google, most of them well acquainted with Python but 
not all of them Python enthusiasts, and I was surprised to see 
unanimity on how PEP 310 *with* __except__ would be a huge step up in 
usefulness wrt the simple __enter__/__exit__ model, which is roughly 
equivalent in power to the C++ approach (destructors of auto variables) 
whose absence from Python and Java some people were bemoaning (which is 
how the whole discussion got started...).

The use cases appear to be aleph-0 or more...;-).  Essentially, think 
of it of encapsulating into reusable forms many common patterns of 
try/except use, much like iterators/generators can encapsulate looping 
and recursive constructs, and a new vista of uses open up...

Imagine that in two or three places in your code you see something 

    ...different blocks here...
except FooError, foo:
    # some FooError cases need whizbang resetting before they propagate
    if foo.wobble > FOOBAR_RESET_THRESHOLD:

With PEP 310 and __except__, this would become:

with foohandler:
    ...whatever block..

in each and every otherwise-duplicated-logic case... now THAT is 

IOW, +1 ... !


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