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Newsgroups: comp.lang.python
Date: Thu, 9 May 2013 04:24:53 -0700 (PDT)
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Subject: Re: Append to python List
From: 88888 Dihedral <dihedral88888@googlemail.com>
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Xref: csiph.com comp.lang.python:45025

Jussi Piitulainen=E6=96=BC 2013=E5=B9=B45=E6=9C=889=E6=97=A5=E6=98=9F=E6=9C=
=9F=E5=9B=9BUTC+8=E4=B8=8B=E5=8D=882=E6=99=8255=E5=88=8620=E7=A7=92=E5=AF=
=AB=E9=81=93=EF=BC=9A
> RAHUL RAJ writes:
>=20
>=20
>=20
> > Checkout the following code:
>=20
> >=20
>=20
> > sample2 =3D [x+y for x in range(1,10) for y in range(1,10) if x!=3Dy]  =
=20
>=20
> > output=3D[]
>=20
> > output=3D[x for x in sample2 if x not in output]
>=20
> >=20
>=20
> > the output I get is
>=20
> > 3 4 5 6 7 8 9 10 3 5 6 7 8 9 10 11 4 5 7 8 9 10 11 12 5 6 7 9 10 11
>=20
> > 12 13 6 7 8 9 11 12 13 14 7 8 9 10 11 13 14 15 8 9 10 11 12 13 15 16
>=20
> > 9 10 11 12 13 14 15 17 10 11 12 13 14 15 16 17
>=20
> >=20
>=20
> > which contains duplicate values.
>=20
>=20
>=20
> The second comprehension, [x for x in sample2 if x not in output], in
>=20
> the context, is equivalent to [x for x in sample2 if x not in []]. It
>=20
> does not refer to an incomplete version of the list that gets assigned
>=20
> to the variable after it's done.

This is just the handy style for a non-critical loop.
In a critical loop, the number of  the total operation counts
does matter in the execution speed.