For review: PEP 308 - If-then-else expression

[Andrew Koenig]

Erik> Well, it's unambiguous, but it could require some extra
Erik> lookahead.  I'm no Python parser expert, but it seems to me the
Erik> only case this happens is when the conditional expression is
Erik> used as a standalone statement, e.g.

Erik> 	if debug: sys.stderr.write('got here\n') else: None

Why is extra lookahead required here?  The "if" is the first token in
the statement, so it can only denote an if-statement, not an
if-expression.  Then we look at the grammar, which says that
an if-statement in this case is

        "if" expression ":" suite "else" ":" suite

and we see that a suite must end in a newline.

This example is therefore a syntax error; one way to rewrite it is:

        if debug: sys.stderr.write('got here\n')
        else: None

Erik> which I must admit is a little perverse in and of itself,
Erik> particularly as an example -- but Guido used it in the PEP so I
Erik> will use it here too.  (No saying, "Duh, just remove the "else:
Erik> None"; I know that perfectly well, and so did Guido.)  I would
Erik> have no problem with, say, requiring parentheses in this one
Erik> case; in all other cases, if you're expecting an expression and
Erik> you see an `if', you know what's up.  (Right?)

If you parenthesize it:

        (if debug: sys.stderr.write('got here\n') else: None)

then the "if" is no longer the first token in the statement, so it
introduces an if-expression.  No problem.

And no lookahead required.  All that's needed is to check whether
the "if" is the first token of its statement.

>> And i'd probably think that

>> if if y<5: yep() else: 0:

>> is the result of some editor-accident :-)

On the other hand, if you drop one of the "if" tokens:

        if y<5: yep() else: 0:

it's a syntax error, and it's still a syntax error if you drop the :
at the end:

        if y<5: yep() else: 0

Erik> But that's true with any form of a conditional operator as the
Erik> conditional expression of an if statement (and quite frankly, in
Erik> any language that has both an if statement and a conditional
Erik> operator); that's just obtuse.  The only thing that makes it
Erik> look like a stutter here is the fact that you see two `ifs' in a
Erik> row, which I think would be almost a benefit for the jackass who
Erik> wrote this code -- right away you know someone did something
Erik> weird instead of having to wade through it.

Hear, hear!

-- 
Andrew Koenig, ark at research.att.com, http://www.research.att.com/info/ark