This is treated extensively in the discussion section of the
iterators-PEP; quoting:
- It has been questioned whether an exception to signal the end of
the iteration isn't too expensive. Several alternatives for the
StopIteration exception have been proposed: a special value End
to signal the end, a function end() to test whether the iterator
is finished, even reusing the IndexError exception.
- A special value has the problem that if a sequence ever
contains that special value, a loop over that sequence will
end prematurely without any warning. If the experience with
null-terminated C strings hasn't taught us the problems this
can cause, imagine the trouble a Python introspection tool
would have iterating over a list of all built-in names,
assuming that the special End value was a built-in name!
- Calling an end() function would require two calls per
iteration. Two calls is much more expensive than one call
plus a test for an exception. Especially the time-critical
for loop can test very cheaply for an exception.
- Reusing IndexError can cause confusion because it can be a
genuine error, which would be masked by ending the loop
prematurely.
I'm not sure why you are reopening this -- special terminating values
are evil IMO. :-)
--Guido van Rossum (home page: http://www.python.org/~guido/)
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