A additional comment (or 2) on my previous message before I go back to
lurk mode.
If the recommended use of each resource template is kept to a single
resource, then each enter and exit can be considered a whole block of
code that will either pass or fail. You can then simplify the previous
template to just:
# open-close pairing
def opening(filename, mode):
def openfile():
f = open(filename, mode)
yield f
f.close()
return openfile
with opening(file1,m),opening(file2,m),opening(file3,m) as f1,f2,f3:
# do stuff with files
The with statement will need to catch any opening errors at the start in
case one of the opening resources fails. Close any opened resources,
then re raise the first exception.
The with statement will also need to catch any uncaught exceptions in
the block, close any opened resources, then re raise the exception.
And again when closing, it will need to catch any exceptions that occur
until it has tried to close all open resources, then re raise the first
exception.
Although it's possible to have more than one exception occur, it should
always raise the first most one as any secondary exceptions may just be
a side effect of the first one.
The programmer has the option to surround the 'with' block with a try
except if he want to catch any exceptions raised.
He should also be able to put try excepts before the yield, and after
the yield, or in the block. (But not surrounding the yield, I think.)
Of course he may cause himself more problems than not, but that should
be his choice, and maybe he thought of some use.
This might be an acceptable use case of try-except in the enter section.
def openfile(firstfile, altfile, mode):
try:
f = open(firstfile, mode)
except:
f = open(altfile, mode)
yield f
f.close()
return openfile
This is still a single open resource (if it succeeds) so should work ok.
Alternate closing could be possible, maybe retrying the close several
time before raising it and letting the 'with' handle it.
Cheers,
_Ron
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