Augmented assignment (was Re: Something in the function tutorial confused me.)

[Roel Schroeven]

OKB (not okblacke) schreef:
> Aahz wrote:
> 
>>>>>>> tup=([],)
>>>>>>> tup[0] += ['zap']
>>>> Traceback (most recent call last):
>>>>   File "<stdin>", line 1, in <module>
>>>> TypeError: 'tuple' object does not support item assignment
> <snip>
>> Obviously, you can easily work around it:
>>
>>>>> t = ([],)
>>>>> l = t[0]
>>>>> l += ['foo']
>>>>> t
>> (['foo'],)
> 
>     	This is quite shocking to me, although after staring at the 
> documentation for a while I guess I understand it.  But it seems to me 
> that the documentation is somewhat misleading on this point, where it 
> says:
> 
>     	"An augmented assignment evaluates the target (which, unlike normal 
> assignment statements, cannot be an unpacking) and the expression list, 
> performs the binary operation specific to the type of assignment on the 
> two operands, and assigns the result to the original target."
> 
>     	This sentence is phrased as though it is the whole story, but it 
> isn't, because the operation might not in fact wind up being an 
> assignment. 

The way I understand this now, the assignment really always is an 
assignment.

1.
   - For immutable objects, the 'binary operation specific to the type of
     assignment on the two operands' doesn't modify any object, but only
     returns a new object which is the result of the operation.
   - For mutable objects that support the inplace-version of the
     operator, the operator modifies the object and then returns it.
3. In both cases that result is assigned to the target. In the second 
case the object was already modified so the assignment doesn't 
accomplish very much, but it still happens.

That explains why "tup[0] += ['zap']" both has an effect (tup[0] now 
contains 'zap') and raises an exception: tup[0] is modified because it 
is a list and lists support inplace addition, but the assignment fails 
because tuples don't support item assignment.

-- 
If I have been able to see further, it was only because I stood
on the shoulders of giants.  -- Isaac Newton

Roel Schroeven