question about try/except blocks

[Steven D'Aprano]

On Thu, 02 May 2013 21:54:29 -0400, J wrote:

> I have this function in a class:
> 
>  def write_file(self, data, dest):
>         with open(dest, 'wb', 0) as outfile:
>             try:
>                 print("IN WRITE_FILE")
>                 outfile.write(self.data)
>             except IOError as exc:
>                 logging.error("Unable to write data to %s: %s", dest,
>                 exc) return False
>             else:
>                 outfile.flush()
>                 os.fsync(outfile.fileno())
>                 return True
[...]
> I think, functionally, that should work, but should nested try/except
> blocks be avoided?


Not particularly. Try/except is cheap to set up, nesting them doesn't 
cost much.

But having said that, your code as given does not protect against rare
but possible errors. For example, just because you can open the file for 
writing doesn't mean you can write to it; just because you can write to 
it doesn't mean flush will succeed; just because flush succeeds doesn't 
mean that syncing to disk will succeed. Any IO operation might fail. 
While you can wrap each one individually, it's probably better to wrap 
the whole lot at once. I'd write it like this:


def write_file(self, data, dest):
    try:
        with open(dest, 'wb', 0) as outfile:
            print("IN WRITE_FILE")
            outfile.write(self.data)
            outfile.flush()
            os.fsync(outfile.fileno())
    except (OSError, IOError) as exc:
        logging.error("Error writing data to %s: %s", dest, exc)
        return False
    return True



File IO is one of the rare exceptions to the idea that try/except blocks 
should wrap the least amount of code possible.



-- 
Steven