Waffling (was Re: single-line terinary operators considered harmful)

[Thomas Wouters]

On Thu, Mar 06, 2003 at 07:28:19PM +0000, Stephen Horne wrote:
> On Thu, 6 Mar 2003 13:03:50 -0700, Steven Taschuk
> <staschuk at telusplanet.net> wrote:

> >Certainly Python's guaranteed evaluation and execution order is
> >very comfortable.  There is the occasional oddity:
> >	d = {}
> >	i = 3
> >	i = d[i] = i+1
> >has a surprising (to me) result.  But this isn't good style anyway.

> I wondered what you were talking about - until I noticed that...

> >>> d={}
> >>> i=3
> >>> i=d[i]=i+1
> >>> i
> 4             <- this was expected
> >>> d[i]
> 4             <- this was expected
> >>> d
> {4: 4}        <- pardon? - why not {3: 4}

> I'm not sure I understand it, but I imagine it has to do with '='
> being n-ary (n >= 2) non-associative (rather than binary
> right-associative). Maybe the assigns are done left-to-right, even
> though the rightmost item is the source and must be evaluated first?

Yes, '=' is a statement, not an expression like in many other languages, and
the chained version is a special case of it, for convenience. The simple
example of 

a = b = c = d

would in C be parsed as

a = (b = (c = d))

and executed innermost-first, but in Python, it's more like

(a, b, c) = (d,)*3

(Not quite, but close enough. Use the 'dis' module to see what it really
does :) This does, however, assign from left to right. But you should not
rely on the order! You could call this a wart, but it doesn't make real
sense to change it the other way 'round either, except if you're thinking of
assignment the wrong way 'round anyway. :) And besides, it has a good side
too.. It stops people from writing code like:

i = d[i] = i+1

Evil-wink'ly y'rs,
-- 
Thomas Wouters <thomas at xs4all.net>

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