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
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