[Python-Dev] Adding a conditional expression in Py3.0

Gareth McCaughan gmccaughan at synaptics-uk.com
Fri Sep 23 14:40:54 CEST 2005

> The reason I like "a if b else c" is because it has the
> most natural word order. In English,
>    My dog is happy if he has a bone, else sad.
> sounds much more natural than
>    My dog is, if he has a bone, happy, else sad.

Neither sounds very natural to me; conditional
expressions don't occur much in English. It's
much more common to hear something like

    My dog is happy if he has a bone; otherwise he's sad.

And that would correspond to a postfix conditional
operator on *statements*, not expressions; Perl and
Ruby have this, but it doesn't seem very Pythonic.

> Interestingly, it looks *more* odd to me if parens are
> included:
>    return (self.arg if self.arg is not None else default)
> I think this is because, without the parens, I tend to read
> the "if" as applying to the whole phrase "return self.arg",
> not just to the "self.arg".

Which is one reason why I *don't* like the proposed
"consequent if antecedent else alternative" syntax;
it looks like a statement modifier when it isn't
really one. When you're forced (by the parens) to
read it correctly, you stop liking it! :-)


Conditional expressions do occur in English, though,
and they do generally have the consequent at the front.
But ... they also generally have the "else-marker"
right at the end. "Buy some pork if it's going cheap,
and some beef if not." "You can divide an angle into
N equal parts with ruler and compasses if N is a power
of 2 times a product of distinct Fermat primes, but
not if N has any other form." "I sleep all night
and work all day."

I don't think a syntax that authentically mirrors
the structure of conditional expressions in English
is likely to be very good.

    x = (123 if p==q; 321 otherwise)
    x = (123 if p==q, else 321 if not)
    x = (123 if p==q; else 321 if r==s; else 999 if not)



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