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

Tim Peters tim.one@home.com
Sun, 14 Oct 2001 15:50:36 -0400

[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