PEP 308: A PEP Writer's Experience - PRO

[Andrew Dalke]

Andrew Koenig
> Here's a different example that you may find more persuasive.
>
> Recall that many programs take file names as parameters, and some of
> those use the convention that if the file name is "-", the program
> should process the standard input instead of opening a file.
 ...
> Then I would like to be able to write this:
>
>     print "Processing", "<stdin>" if filename == "-" else filename
>
> You might claim that it is better to write
>
>     print "Processing",
>     if filename == "-":
>         print "<stdin>"
>     else:
>         print filename

Actually, you still need to open the file, correct?  If you use
the if/else above then you need to check the extension twice,
which seems more fragile in the face of maintainence.  Instead,
it should be

  if filename == "-":
    displayname = "<stdin>"
    infile = sys.stdin
  else:
    displayname = filename
    infile = open(filename)
  ....
  print "Processing", displayname

However, alternatives are

  infile = sys.stdin if filename = "-" else open(filename)
   ...
  print "Processing", infile.name

This works because sys.stdin.name == '<stdin>'.  But
supppose you had to have "*stdin*".  Then the following
works too

  infile, displayname = sys.stdin, "*stdin*" if filename == "-" else
open(filename), filename

Err, but this may need some parens as well for precedence.

  infile, displayname = (sys.stdin, "*stdin*") if filename == "-" else
(open(filename), filename)

Do you consider this a use or misuse of the if/else expression?
I happen to think it's somewhat harder to understand then

  if filename == "-":
    displayname = "*stdin*"
    infile = sys.stdin
  else:
    displayname = filename
    infile = open(filename)

or even the more terse

  if filename == "-":
    displayname, infile = "*stdin*", sys.stdin
  else:
    displayname, infile = filename, open(filename)


> Of course, I don't consider this argument to be proof.  There isn't
> going to be any proof in this discussion, because the whole question
> is *always* about which of two equivalent forms to prefer.

Yep.

                    Andrew
                    dalke at dalkescientific.com