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Date: Fri, 13 Jan 2012 14:26:09 -0800 (PST)
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Subject: Re: copy on write
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:18951

Ethan Furman=E6=96=BC 2012=E5=B9=B41=E6=9C=8814=E6=97=A5=E6=98=9F=E6=9C=9F=
=E5=85=ADUTC+8=E4=B8=8A=E5=8D=882=E6=99=8240=E5=88=8647=E7=A7=92=E5=AF=AB=
=E9=81=93=EF=BC=9A
> Steven D'Aprano wrote:
> > Normally this is harmless, but there is one interesting little glitch y=
ou=20
> > can get:
> >=20
> >>>> t =3D ('a', [23])
> >>>> t[1] +=3D [42]
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> > TypeError: 'tuple' object does not support item assignment
> >>>> t
> > ('a', [23, 42])
>=20
>=20
> There is one other glitch, and possibly my only complaint:
>=20
> --> a =3D [1, 2, 3]
> --> b =3D 'hello, world'
> --> a =3D a + b
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: can only concatenate list (not "str") to list
> --> a +=3D b
> --> a
> [1, 2, 3, 'h', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd']
>=20
> IMO, either both + and +=3D should succeed, or both should fail.
>=20
> ~Ethan~

The +=3D operator is not  only for value types in the above example.=20
=20
An operator of two operands and an operator of three operands  of=20
general object types are two different operators.=20