Compound Assignment Operators ( +=, *=, etc...)

[Tim Peters]

>> ... does
>>
>>     z = x
>>     x += y
>>     print z
>>
>> mean
>>
>>     x = x + y   # x is now bound to an entirely different object
>>     # and the old x is printed
>>
>> or
>>
>>    x.extend(y)   # x is still the same object, but mutated
>>    # and the new x is printed
>>
>> ?  The resulting *values* compare equal either way, but there are
>> differences in efficiency and effects on object identity.

[Andrew McDowell]
> I don't see where the "non-obviousness" comes into play...
> (maybe that's why it's not obvious <wink>)

In that reasonable people can (& do) expect either behavior.

> Why would you confuse x += y (which would seem to imply the
> former) to mean x.extend(y) ?

Efficiency; and, to half the reasonable people in the world, obviousness
<wink>.

> It _is_ and assignment statement after all..

That's my mild preference, and it's easier to implement that way; but some
reasonable people want it the other way.  Consider the NumPy extension,
where "x" can denote a matrix with millions of elements and consuming many
Mb of storage.  There you may desperately not want e.g.

    x += 1

to create an entirely new similar-sized matrix, yet still want the economy
of expression.  Guido's preferred scheme caters to either behavior.

> Not to touch on a sore topic, but hop over to comp.lang.perl and
> check out the (rather long and heated) comparison of perl and python.

So *that's* what it is -- I thought it was a flame war.

> The lack of these statements is causing quite a bit of a stir.

I expect you could post "have a nice day!" to that thread and also "cause"
quite a bit of a stir.  Flame wars aren't "about" the topics being toasted,
y'know <0.1 wink>.

>> ... it's up to x.__addeq__(y) to decide what "+=" (& so on) means.

> Why addeq??  Why not just add?

So that it's *possible* for the implementation of x's type to arrange for

    y = x + 1

to create a new object but for

    x += 1

not to.  If you don't want to allow for mutation, __addeq__ isn't needed
(and you can find an implementation sketch in DejaNews for this case -- it's
easy to implement in Python's front end).

lettuce-should-never-be-chewed-twice-ly y'rs  - tim