[Python-ideas] Augmented assignment syntax for objects.

[Nathaniel Smith]

On Tue, Apr 25, 2017 at 3:30 PM, Erik <python at lucidity.plus.com> wrote:
> On 25/04/17 23:05, Paul Moore wrote:
>>
>> 1. Writing out the assignments "longhand" is an unacceptable burden.
>
>
> There are reasons why augmented assignment was implemented. One of them was
> to make the code easier to read:
>
>   foil = foil + 1
>   foil = foi1 + 1
>   foil += 1
>
> Should one be silly enough to have a "foil" and "foi1" variable in scope,
> only one of those is clearly incrementing a variable without requiring a
> slightly harder look ;)

But if this were the only argument for +=, then I'm not sure it would
have ever been added :-).

The really compelling cases for += are things like:

foo.some_attr[get_the_key(kwarg=something)] =
foo.some_attr[get_the_key(kwarg=something)] + 1

vs

foo.some_attr[get_the_key(kwarg=something)] += 1

where the latter is *much* more readable, and it only calls
get_the_key once, which (depending on what it does) may be both a
substantial efficiency win and also guarantees this is actually an
increment – we don't have to read get_the_key's source code first to
figure out if the two calls actually return the same value.

Another example:

arr = arr + 1
arr += 1

If 'arr' is a numpy array then these actually do very different
things: the first one allocates a new array and then rebinds the name
'arr' to the copy; the second modifies the array object in place. The
former is usually what you want, but without augmented assignment you
have to remember to do awkward things like 'arr[...] = arr + 1'. And
it gets worse: since 'arr[...] = arr + 1' has to make a copy, it's
about twice as slow as 'arr += 1'. The real equivalent of 'arr += 1'
is 'np.add(arr, 1, out=arr)', which is *really* bad.

Are there any similar arguments for .=?

-n

-- 
Nathaniel J. Smith -- https://vorpus.org