PEP 308: "then" "else" for deprecating "and" "or" side effects
mso at oz.net
Tue Feb 18 21:23:57 CET 2003
The tertiary operator is one of those long-requested features that's
not going to go away, because it's genuinely useful and allows one to
eliminate an entire if-block, the way a list comprehension eliminates
an entire for-block. Rejected or not, the tertieary operator will
continue to be a FAQ, so we may as well put it in the language now so
the issue will go away. That's what we did for '+=' and string
methods, which were rejected for years for pedantic reasons (+= as
unnecessary, string methods because "immutable objects don't have
methods"). Python has always been great in adding, not every feature,
but a few well-chosen features that have wide applicability. The
tertiary operator would be a good complement. (Sets and dict
comprehensions are two others; fortunately, sets are already here. :)
Christian Tismer <tismer at tismer.com> wrote in message news:<mailman.1045192739.6092.python-list at python.org>...
> "and" and "or" are two-headed beasts:
> At first glance, they pretend to be logical operators.
> At second inspection, they do this logic by promoting
> one of their operands.
... which leads to the frustrating situation that they are "almost" a
tetiary operator but with a sneaky little gotcha.
result = a and b or c
# Whoops, 'b' is false. My result is wrong!
The fact that people try to use 'and ... or' so broadly for this shows
that something is missing. Only one little tweak is needed: an
alternative construct that faithfully returns 'b' if 'b' is false.
> Due to the presence of a real boolean type, we
> should ask ourselves if "and" and "or" shouldn't be
> deprecated in their current meaning, towards being
> "boolean only"?
> I think it would be nicer if "and" and "or" would only
> produce true or false. But this would break lots of code.
'and' and 'or' are fine as they are.
result = replyVar or DEFAULT
# I want DEFAULT if replyVar is '', 0 or None. What's wrong
# with that? If it ain't broke, don't fix it.
result = conditionA and conditionB
# A true boolean. Who cares what exactly 'result' is as long as
# it works properly in an 'if' statement. One can always use
# bool(result) if one really wants 'True'.
Now, on to syntaxes. I like these:
1) result = b if a else c
# Advantage: parallel to list comprehension syntax.
2) result = a ? b : c
# Similar to C/Perl/PHP. This is arguably more readable
# than (1).
3) result = a ?? b || c
# Good visual separation.
# Disadvantage: irritating to C/Perl programmers, for
# whom these operators have an entirely different meaning.
4) result = a then b else c
I don't like these:
A) result = if a then b else c
# Too polymorphic. Let's avoid expressions that "almost" look
# like statements, for the same reason '=' is not allowed in
B) result = if (a) then (b) else (c)
# This would be the only time Python requires '()' except for
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