Python dot-equals (syntax proposal)

[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.

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).

-tkc