[Python-Dev] conditional expressions? (original) (raw)

Tim Peters tim.one@home.com
Mon, 15 Oct 2001 19:30:58 -0400

||| ... ||| Some examples where parentheses are required: |||||| (if 1 then 2 else 3) + 4

|| But that's because it's a top-level expression beginning with "if", not || because of the "+", right? In the patched Python, this works fine: |||| >>> x = if 0 then 2 else 3 + 4 || >>> x || 7 || >>>

| But it groups differently: the 'else' part is '3 + 4' here. Try 'if | 1'. :-)

Yes, I understand how it groups, and am fine with it. The question I asked is the one I asked : the paretheses aren't required because of the "+", but solely because it's a "top-level expression". This is by way of contrast with the example that was next to it in the original:

a[(if i then 2 else 3) : 4]

where, unlike the "+" example, the parens are also required if you stick that on the RHS of an assignment.

I thought both were intended to be examples of:

you need to add parentheses to disambiguate expressions:
...
- if the conditional expression is to be combined with a unary or
  binary operator.

as "+" and ":" both feel like binary operators in these examples.

|| OTOH, these seem odd: |||| >>> 4 + if 1 then 2 else 3 || File "", line 1 || 4 + if 1 then 2 else 3 || ^ || SyntaxError: invalid syntax

That's too much work for the parser to get right. And you'd get anomalies like

3 * if 1 then 2 else 3 + 4 -- how should this group?

That's a good point.

... Because the conditional ends in something that "pulls in" all following binary operators and their operands until it hits a ")", I find it safer not to let it accept an operator at the front.

I agree. The "precedence pull" of "*" does make it more tempting to misread

x = 3 * if 1 then 2 else 3 + 4

than to misread

x = if 1 then 2 else 3 + 4

and I'm personally happier with "excess" parentheses anyway.