Python dot-equals (syntax proposal)

[Alf P. Steinbach]

On 01.05.2010 14:13, * Tim Chase:
> On 05/01/2010 12:08 AM, Patrick Maupin wrote:
>> +=, -=, /=, *=, etc. conceptually (and, if lhs object supports in-
>> place operator methods, actually) *modify* the lhs object.
>>
>> Your proposed .= syntax conceptually *replaces* the lhs object
>> (actually, rebinds the lhs symbol to the new object).
>
> The += family of operators really do rebind the symbol, not modify the
> object.
>
>  >>> from decimal import Decimal
>  >>> d = Decimal(42)
>  >>> e = Decimal(18)
>  >>> orig = d
>  >>> d += e
>  >>> d
> Decimal("60")
>  >>> e
> Decimal("18")
>  >>> orig
> Decimal("42")
>  >>> d is orig
> False
>
> If your suggestion that += *modifies* the object, then orig would now
> unintuitively contain 60 and "d is orig" would return True.

In some cases += modifies the object. For CPython this is an optimization for 
the 'str' type, reducing to O(n) time the common newbie O(n^2) loops. The 
criterion for doing it is that there is exactly 1 reference (as is the case 
after a first append, subsequent appends can just modify).


> This doesn't preclude you from implementing a self-mutating += style
> __add__ method and returning "self", but it's usually a bad idea unless
> it's dire for performance (and even then, think it over a couple times).

Agreed, at the Python level one doesn't in general have the necessary 
information to do it safely. Nothwithstanding the current CPython and Jython 
documentation error of sys.getrefcount (or whatever the name was) that indicates 
that it's available in any implementation.


Cheers,

- Alf