[Python-Dev] conditional expressions? (original) (raw)

Guido van Rossum guido@python.org
Mon, 15 Oct 2001 17:42:39 -0400

[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

[Tim]

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,)).

These don't bother me any more than

x = 2,

does.

> ... > 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 >>>

But it groups differently: the 'else' part is '3 + 4' here. Try 'if 1'. :-)

OTOH, these seem odd:

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

That's too much work for the parser to get right. And you'd get anomalies like

3 * if 1 then 2 else 3 + 4

-- how should this group?

>>> x = 4 + if 1 then 2 else 3 File "", 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.

Only because the examples are so simply.

> 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.

They act the same as lambda in this respect.

Because the conditional ends in something that "pulls in" all following binary operators and their operands until it hits a ")", I find it safer not to let it accept an operator at the front.

--Guido van Rossum (home page: http://www.python.org/~guido/)