[Python-Dev] conditional expressions?

[Tim Peters]

[Guido]
> ...
> Some examples where no parentheses are needed (note that a comma binds
> less tight than a conditional expression -- same as for lambda):
>
>     x = if 1 then 2 else 3, y

Also

    x = if 1 then 2 else 3,

which sets x to the singleton tuple (2,).  This is an old potential
confusion, though, much the same as the current

    x = 2 or 3,

(which also sets x to (2,)).

> ...
> Some examples where parentheses *are* required:
>
>     (if 1 then 2 else 3) + 4

But that's because it's a top-level expression beginning with "if", not
because of the "+", right?  In the patched Python, this works fine:

>>> x = if 0 then 2 else 3 + 4
>>> x
7
>>>

OTOH, these seem odd:

>>> 4 + if 1 then 2 else 3
  File "<stdin>", line 1
    4 + if 1 then 2 else 3
         ^
SyntaxError: invalid syntax

>>> x = 4 + if 1 then 2 else 3
  File "<stdin>", line 1
    x = 4 + if 1 then 2 else 3
             ^
SyntaxError: invalid syntax
>>>

I say "odd" because there's no question about what they mean.

>     a[(if i then 2 else 3) : 4]
>
> In some situations I'm not sure what's right; The un-parenthesized
> form looks weird although it's not neede to avoid ambiguity:
>
>     if (if 1 then 2 else 3): pass
>     print (if 1 then 2 else 3)

OTOH,

    print >> sys.stderr, if 1 then 2 else 3
and
    print 4, if 1 then 2 else 3

don't look particularly weird but also raise SyntaxError.

As a matter of taste, I was most surprised by the

    4 + if 1 then 2 else 3

examples.