Re: [Python-Dev] Explicitly declaring expected exceptions for a block

22 Jun 2005

      ...
It was possible in PEP 340 and in early drafts of PEP 346, but it
isn't possible in PEP 343.
...
In PEP 343, the statement template *cannot* suppress exceptions - it
can react to them, and it can turn them into different exceptions, but
that's all.
...doing homework...

The following code, combined from PEP 343 and Mr. Du Bois suggestion:

--------------------------------------------------------------

from sys import exc_info

class ignored_exceptions(object):
        def __init__(self, *exceptions):
                self.exceptions = exceptions
        def __enter__(self):
                return None
        def __exit__(self, type, value, traceback):
                try:
                        raise type, value, traceback
                except self.exceptions:
                        pass

try:
        with = ignored_exceptions(TypeError)
        with.__enter__()
        try:
                try:
                        raise TypeError()
                except:
                        exc = exc_info()
                        raise
        finally:
                with.__exit__(*exc)
except:
        print exc_info()

--------------------------------------------------------------

still yields exceptions.TypeError.

Now, back to original question then, do you think it'd be
beneficial to have some sort of exception ignoring or expecting
statement ?

Sincerely,
Dmitry Dvoinikov
http://www.targeted.org/

--- Original message follows ---
...
Paul Du Bois wrote:
...
If I understand PEP 343 correctly, it allows for easy implementation
of part of your request. It doesn't implement the else: clause, but
you don't give a use case for it either.
class ignored_exceptions(object):
   def __init__(self, *exceptions):
       self.exceptions = exceptions
   def __enter__(self):
       return None
   def __exit__(self, type, value, traceback):
       try:
           raise type, value, traceback
       except self.exceptions:
           pass
with ignored_exceptions(SomeError):
   do stuff
I don't see the use, but it's possible.
...
It was possible in PEP 340 and in early drafts of PEP 346, but it 
isn't possible in PEP 343.
...
In PEP 343, the statement template *cannot* suppress exceptions - it
can react to them, and it can turn them into different exceptions, but
that's all.
...
And yes, this is deliberate - the control flow is too murky otherwise:
...
with some_template():
       raise TypeError
   print "Hi!"
...
Does that code print "Hi!" or not? Under PEP 343, you know it doesn't,
because the TypeError isn't trapped. If templates could actually 
suppress exceptions, you'd have no idea what the code does without 
looking up the definition of 'some_template' - this is a bad thing, 
which is why PEP 343 defines the semantics the way it does.
...
However, I'm with Michael - every time I've needed something like 
this, I have had non-trivial code in either the 'except' or the 'else'
blocks of the try statement.
...
Regards,
Nick.