[Python-Dev] conditional expressions (RE: Loop-and-a-half (Re: Curious assignment behaviour))

[Tim Peters]

[Tim]
> If people sign off on taking "then" as a new keyword, I think
> the chances are good that we could get
>
>     x = if e1 then e2 else e3
>
> into 2.2b1.  That's the only obvious spelling, hence the only
> truly Pythonic way to spell it.  Other languages spelling it that
> way range from Algol-60 (Guido's first intense language affair) to
> Haskell.

[Paul Rubin, among others of similar mind]
> This sounds fine to me.

Alas, it didn't to Python's parser -- one-token lookahead isn't enough to
distinguish

    if 1:

from

    if 1 then 2 else 3

let alone

    if a + b / c:

from

    if a + b / c then 2 else 3

etc.

and Python won't grow anything a simple parser can't sort out.

Everything's cool if parens are required around a conditional expression,
though, in which case:

    x = if e1 then e2 else e3 + 1   # SyntaxError
    x = (if e1 then e2 else e3) + 1 # cool
    x = (if e1 then e2 else e3 + 1) # cool
    x = if e1 then e2 else e3       # SyntaxError
    x = (if e1 then e2 else e3)     # cool
    x = if if e1 then e2 else e3 then e4 else e5     # SyntaxError
    x = (if (if e1 then e2 else e3) then e4 else e5) # cool

Seems a mixed bag, but I'm more interested in readability and the
functionality than in minimizing keystrokes; requiring parens doesn't hurt
the goals I care about.

implemented-but-not-checked-in-ly y'rs  - tim