Python dot-equals (syntax proposal)

[Stefan Behnel]

Tim Chase, 01.05.2010 14:13:
> 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.
>
> 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).

It's not like this was unprecedented in Python, though:

   Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41)
   [GCC 4.4.3] on linux2
   Type "help", "copyright", "credits" or "license" for more information.
   >>> l = [1,2,3]
   >>> a = l
   >>> l += [4,5,6]
   >>> l
   [1, 2, 3, 4, 5, 6]
   >>> a
   [1, 2, 3, 4, 5, 6]

And I'm pretty sure this wasn't just done for performance reasons. Mutable 
data types behave that way.

Stefan