[Python-Dev] Adding a conditional expression in Py3.0 (original) (raw)

Terry Reedy tjreedy at udel.edu
Sat Sep 24 05:21:14 CEST 2005

"Greg Ewing" <greg.ewing at canterbury.ac.nz> wrote in message news:433223A8.3010400 at canterbury.ac.nz...

Terry Reedy wrote:

Many people, perhaps most, including me, read

exp1 if exp2 else exp3 # as cond if etrue else efalse # in direct analogy with cond ? etrue : efalse # from C I'd have thought only Forth programmers would be prone to that!

Never wrote a single line, and may have not read one (in DDJ? Byte?) for 15-20 years. How do you read such C? Cond 'Qmark' ....?

It would surprise me greatly if it's really true that most people would read it that way.

OK. I withdraw 'possibly most' as not directly relevant. It would not take that many people to have enough people mistakenly write or read it that way for this construct to be somewhat as bug prone, in practice, as and/or.

During the c.l.p debate, someone counted about 100 correct uses of 'a and b or c' in the standard library. But one real misuse edged Guido toward replacing it. So I think the replacement should be as clear as reasonably possible and clearly an improvement.

Especially given that, in real code, you're not going to be looking at abstract names like exp1, exp2, exp3, but (hopefully) something a lot more meaningful.

Because every object in Python (ignoring buggy or unusual nonzero methods) has a boolean value, any expression can be a conditional expression and hence conditional expressions often lack a boolean operator to flag the expression as the likely conditional.

Can you honestly say that you would read

Given that setup, how could I possible misread :-?

return redvalue if color == 'red' else bluevalue as if redvalue: return color == 'red' else: return bluevalue ?

I can imagine that if I were to read the return naively, I might wonder whether the expressions were reversed. And certainly, not all code I have read has had such obvious names.

Now, can you honestly say that you would (naively) read

return foo if bar else baz

and be certain you knew what it meant?

Terry J. Reedy

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