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Sun Jan 26 18:16:30 CET 2014 · https://mail.python.org/pipermail/python-dev/2014-January/132102.html

On 01/26/2014 09:59 AM, francis wrote:
> On 01/26/2014 03:42 AM, Raymond Hettinger wrote:
>>
>> I think that is an over-simplification.  The argument unpacking was handy
>> in a number of situations where *args wouldn't suffice:
>>
>>    lambda (px, py), (qx, qy): ((px - qx) ** 2 + (py - qy) ** 2) ** 0.5
>>
>> IIRC, the original reason for the change was that it simplified the
>> compiler a bit,
>> not that it was broken or not useful.
> I'm not sure if that's applicable or other issues arise with:
> 
> def fn(*p): px,py,qx,qy = p; return ((px - qx) ** 2 + (py - qy) ** 2) **
> 0.5

[Dropping some whitespace to get things to all fit on one line]

The goal is to call fn as:
fn((1, 1), (2, 2))

So, in 2.7:
>>> def fn((px, py), (qx, qy)):
...   return ((px-qx)**2+(py-qy)**2)**0.5
...
>>> fn((1, 1), (2, 2))
1.4142135623730951
>>>

The nearest equivalent in 3.3 (also works in 2.7):
>>> def fn(p, q):
...   (px, py), (qx, qy) = p, q
...   return ((px-qx)**2+(py-qy)**2)**0.5
...
>>> fn((1, 1), (2, 2))
1.4142135623730951

For a lambda in 3.3, you're out of luck because you can't do the
assignment. There, the best you can do is:
>>> fn = lambda p, q: ((p[0]-q[0])**2+(p[1]-q[1])**2)**0.5
>>> fn((1, 1), (2, 2))
1.4142135623730951

Which isn't quite so readable as the equivalent lambda in 2.7:
>>> fn = lambda (px, py),(qx, qy):((px-qx)**2+(py-qy)**2)**0.5
>>> fn((1, 1), (2, 2))
1.4142135623730951

As someone pointed out, it's not quite the same when you do your own
tuple unpacking, but it's probably close enough for most cases.

Eric.

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