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Newsgroups: comp.lang.python
Date: Tue, 12 Feb 2013 18:43:42 -0800 (PST)
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Subject: Re: Implicit conversion to boolean in if and while statements
From: 88888 Dihedral <dihedral88888@googlemail.com>
To: comp.lang.python@googlegroups.com
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Xref: csiph.com comp.lang.python:38799

Rick Johnson=E6=96=BC 2013=E5=B9=B42=E6=9C=8813=E6=97=A5=E6=98=9F=E6=9C=9F=
=E4=B8=89UTC+8=E4=B8=8A=E5=8D=881=E6=99=8248=E5=88=8607=E7=A7=92=E5=AF=AB=
=E9=81=93=EF=BC=9A
> On Monday, February 11, 2013 11:55:19 PM UTC-6, Chris Angelico wrote:
>=20
> > On Tue, Feb 12, 2013 at 12:06 PM, 88888 Dihedral wrote:
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> > > A permanently mutated list is a tuple of constant objects.
>=20
> >
>=20
> > I nominate this line as "bemusing head-scratcher of the week".
>=20
>=20
>=20
> Actually the statement is fact IF you can grok it through the eyes of cla=
rity.
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>=20
>=20
> "A permanently mutated list..."
>=20
>=20
>=20
> A list that has been mutated permanently, that is, it cannot be changed b=
ack into a list. psst: i have a sneaking suspicion that he his referring to=
 tuples, let's see.
>=20
>=20
>=20
> "...is a tuple..."
>=20
>=20
>=20
> Ha! Well in Python the immutable sequence type /is/ a tuple after all.
>=20
>=20
>=20
> "...of constant objects..."
>=20
>=20
>=20
> The tuple contains objects, and it's objects will maintain a constant ord=
ering (relatively in tuple structure) until until the tuple's death.=20
>=20
>=20

>>> a1=3D[1,2,3]
>>> tuple1=3D(a1,4,5,6)
>>> tuple1
([1, 2, 3], 4, 5, 6)
>>> a1=3D[1,2]
>>> tuple1
([1, 2, 3], 4, 5, 6)
>>>=20

Yes, a tuple of constant objects is still not clear.=20


>=20
> Your confusion may stem from interpreting "constant" as the CS term "CONS=
TANT"[1]; whereby the objects in the tuple are programming CONSTANTS, that =
is, unable to change. But in reality, although a tuple (bka:StaticList) can=
not expand to add more objects, or shrink to eject existing objects, the ob=
jects themselves CAN change their own internal state WITHOUT disrupting the=
 immutable harmony of the tuple.=20
>=20
>=20
>=20
> Observe:
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>    =20
>=20
> py> class Foo(object):
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> 	pass
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> py> foo =3D Foo()
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> py> t =3D (1,'1', foo)
>=20
> py> t
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> (1, '1', <__main__.Foo object at 0x0267BF50>)
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> py> t[-1].bar =3D "abc"
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> py> t
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> (1, '1', <__main__.Foo object at 0x0267BF50>)
>=20
>=20
>=20
> Or by expanding a list
>=20
>=20
>=20
> py> t =3D (1,2,3)
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> py> t =3D t+([],)
>=20
> py> t
>=20
> (1, 2, 3, [])
>=20
> py> t[-1].append('circus')
>=20
> py> t
>=20
> (1, 2, 3, ['circus'])
>=20
>=20
>=20
> [1] Which is an unfortunate side-effect of polysemy and compounded expone=
ntially by naive (and sometimes a purely malevolent intent when) transforma=
tion of words into esoteric problem domains.