Trying to understand += better

[Terry Reedy]

Roy Smith wrote:
> In article <4b0a01aa$1 at dnews.tpgi.com.au>, Lie Ryan <lie.1296 at gmail.com> 
> wrote:
> 
>> The semantic of the in-place operator is something like:
>> x += y
>> becomes
>> x = x.__iadd__(y)

Except that the expression x is evaluated just once instead of twice.

>> thus
>> foo.bar += baz
>> becomes
>> foo.bar = foo.bar.__iadd__(baz)
>>
>> So the call sequence is,
>> foo.__getattr__('bar') ==> x
>> x.__iadd__(baz) ==> y
>> foo.__setattr__('bar', y)
> 
> I don't get where the __setattr__() call comes from in this situation.

Augmented *ASSIGNMENT* is a type of assignment.

The dis module can be used to see what CPython does.
 >>> from dis import dis
 >>> def f():
	 foo.bar += baz

 >>> dis(f)
   2           0 LOAD_GLOBAL              0 (foo)
               3 DUP_TOP
               4 LOAD_ATTR                1 (bar)
               7 LOAD_GLOBAL              2 (baz)
              10 INPLACE_ADD
              11 ROT_TWO
              12 STORE_ATTR               1 (bar)
...
This amounts to what Roy said, with x and y being temporary entries on 
the stack.

Terry Jan Reedy