[Tutor] What's the best way to ask forgiveness here?

[Brian Jones]

I've been coding Python long enough that 'asking forgiveness instead of
permission' is my first instinct, but the resulting code is sometimes
clumsy, and I wonder if someone can suggest something I'm missing, or at
least validate what's going on here in some way.

What I'm trying to do is write a file to a directory. However, the directory
may not exist the first time I try to write a file there, so I'm going to
first try to write the file, and if I get an exception, create the directory
(er, *try* to), and *then* write the file there. Here's my first shot at the
code:

        try:
            self.save_file(picfile_fullpath, picdata)
        except IOError as err:
            # directory doesn't exist. Try to create it.
            try:
                os.makedirs(picfile_fullpath)
            except OSError as oserr:
                logging.error("Can't create file path: %s (%s)" %
(picfile_fullpath, oserr))
            else:
                # Created dir, now write file.
                try:
                    self.save_file(picfile_fullpath, picdata)
                except IOError as err:
                    logging.error("Bailing. Couldn't save file %s (%s)" %
(picfile_fullpath, err))
                    return False

Doesn't this seem less readable than the 'ask permission' equivalent? I
think it does, but in this case asking permission for every single operation
when the dir will only need to be created a single time (and then may be
written to several hundred times) is pretty wasteful.

I suppose I could set some sentinel variable and check for it in a while
loop, but then I need some other scaffolding code to make sure I don't
infinitely loop trying to create the directory, and probably some other
stuff I'm forgetting, so it strikes me as being just as messy.

Is there a clean sort of pattern to apply in instances like this?

Thanks.
brian



-- 
Brian K. Jones
My Blog          http://www.protocolostomy.com
Follow me      http://twitter.com/bkjones
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