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

Steve Holden steve at holdenweb.com
Tue Mar 7 16:29:18 CET 2006

Jim Jewett wrote:

On 3/7/06, Paul Moore <p.f.moore at gmail.com> wrote:

The parentheses around genexps were (AFAICT) different - without them, the grammar was ambiguous, so some way of disambiguating was needed. The out-of-order evaluation is a very large change, because now we have a situation where normal parsing completes an expression, but needs to avoid evaluating it, just in case. Currently, we can write: >>> if False: >>> print r >>> else: >>> print 6 6 >>> r Traceback (most recent call last): File "<pyshell#14>", line 1, in -toplevel- r NameError: name 'r' is not defined In the above example, r doesn't get evaluated because the if ahead of it says to skip that branch. But with conditional expressions, that flow control is changed from later in the program. I don't think we'll see the equivalent of Intercal Suck Points anywhere but intentionally obfuscated code, but I do expect to see: >>> sideeffect() if condition In fact, I think the below examples are reasonable uses that do a better job of expressing intent than the if statement would. I just don't like the mental backtrack they require, and would like some sort of advance warning. Parentheses at least tell me "You're not done yet; keep reading." >>> ack(r,r) if r not in cache >>> log(message) if errorflag I think you've misunderstood the nature of the change. HTe new conditionals are expressions, they aren't postfix conditionals for statements.

regards Steve

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