Specifying the '+' more than once while opening a file results in the
error "Must have exactly one of read/write/append mode". The attached
patch extends that with ".. and at most one optional plus".

Further, the patch checks these after the loop that parses the mode
string, avoiding some unnecessary gotos and saving a few lines of code
overall.

On trunk, it seems that it's perfectly happy if you specify more than 
one '+':

Python 2.7a0 (trunk, May 29 2009, 05:57:26) 
[GCC 4.0.1 (Apple Inc. build 5470) (Aspen 5470.3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> open('foo.txt', 'w++')
<open file 'foo.txt', mode 'w++' at 0x39b2a0>

Is this still really an issue then?  The current trunk also allows 
multiple mode letters too, so it seems like a decision has been made.

Good catch, it just took me a while to actually figure out myself where
the bug is. Try the following instead:

  import io
  io.FileIO('foo.text', 'w++')

This will yield "ValueError: Must have exactly one of read/write/append
mode" with 2.6 on win32. I haven't tested on the latest 2.x or 3.x
branches, but looking at the 2.7 branch, I see the faulty code is still
there.

BTW:
Using io.open('foo.text', 'w++') yields "ValueError: invalid mode:
'w++'", I would have expected the same error as for io.FileIO() above.
Using open('foo.text', 'w++') works.
Using open('foo.text', 'ww++') yields "IOError: [Errno 22] invalid mode
('ww++') or filename: 'foo.text')".

In other words, Python 2.6 is behaving a bit inconsistent here. The
patch only fixes one of those issues, the others and the necessary unit
tests remain.

The offending lines in io.py are:
    modes = set(mode)
    if modes - set("arwb+tU") or len(mode) > len(modes):
        raise ValueError("invalid mode: %r" % mode)

In particular, the "or len(mode) > len(modes)" is picking off the fact 
that there is repeated mode characters.  Leaving that off allows 
io.open() to behave exactly like the built-in open() call.

OTOH, someone obviously wanted to make sure that repeat mode characters 
were not allowed.  So I guess someone needs to rule on whether we want 
io.open() and io.FileIO() to behave like the built-in, or to keep things 
more strict.

Amended error message in r85773.