exception handling in complex Python programs

[dbpokorny at gmail.com]

On Aug 19, 4:12 pm, Steven D'Aprano <st... at REMOVE-THIS-
cybersource.com.au> wrote:
> On Tue, 19 Aug 2008 11:07:39 -0700, dbpoko... at gmail.com wrote:
> >   def do_something(filename):
> >     if not os.access(filename,os.R_OK):
> >       return err(...)
> >     f = open(filename)
> >     ...
>
> You're running on a multitasking modern machine, right? What happens when
> some other process deletes filename, or changes its permissions, in the
> time after you check for access but before you actually open it?

This is a good point - if you want to use the correct way of opening
files, and
you don't want to worry about tracking down exception types, then we
can probably
agree that the following is the simplest, easiest-to-remember way:

  def do_something(filename):
    try:
      f = open(filename)
    except:
      <handle exception>
    ...

Opening files is a special case where EAFP is the only correct
solution (AFAIK). I still liberally sprinkle LBYL-style "assert
isinstance(...)" and other similar assertions in routines. The point
is that EAFP conflicts with the interest of reporting errors as soon
as possible (on which much has been written see, for instance Ch. 8 -
Defensive Programming in Code Complete), but LBYL conflicts with
correctness when objects can be shared.

Also, look at the man page for access. I have found at least two (one
on my Linux box, another online) that essentially say "never use it."
I completely forgot about this in my last post...

David