copy on write

[88888 Dihedral]

Ethan Furman於 2012年1月14日星期六UTC+8上午2時40分47秒寫道：
> Steven D'Aprano wrote:
> > Normally this is harmless, but there is one interesting little glitch you 
> > can get:
> > 
> >>>> t = ('a', [23])
> >>>> t[1] += [42]
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> > TypeError: 'tuple' object does not support item assignment
> >>>> t
> > ('a', [23, 42])
> 
> 
> There is one other glitch, and possibly my only complaint:
> 
> --> a = [1, 2, 3]
> --> b = 'hello, world'
> --> a = a + b
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: can only concatenate list (not "str") to list
> --> a += b
> --> a
> [1, 2, 3, 'h', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd']
> 
> IMO, either both + and += should succeed, or both should fail.
> 
> ~Ethan~

The += operator is not  only for value types in the above example. 
 
An operator of two operands and an operator of three operands  of 
general object types are two different operators.