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Date: Mon, 9 Jan 2012 15:58:41 -0800
Subject: Re: Explanation about for
From: Chris Rebert <clp2@rebertia.com>
To: =?UTF-8?B?zp3Ouc66z4zOu86xzr/PgiDOms6/z43Pgc6xz4I=?= <nikos.kouras@gmail.com>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:18722

On Mon, Jan 9, 2012 at 3:23 PM, =CE=9D=CE=B9=CE=BA=CF=8C=CE=BB=CE=B1=CE=BF=
=CF=82 =CE=9A=CE=BF=CF=8D=CF=81=CE=B1=CF=82 <nikos.kouras@gmail.com> wrote:
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D
> dataset =3D cursor.fetchall()
>
> for row in dataset:
> =C2=A0 =C2=A0print ( "<tr>" )
>
> =C2=A0 =C2=A0for item in row:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0print ( "<td><b><font color=3Dyellow> %s </td>=
" % item )
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D
>
> and this:
>

Your second snippet makes use of Python's
iterable/sequence/tuple-unpacking feature. Here's the relevant part of
the Language Reference:
http://docs.python.org/reference/simple_stmts.html#assignment-statements

By way of example, given:
seq =3D [1,2,3,4]
Then:
w, x, y, z =3D seq
Results in:
w =3D 1
x =3D 2
y =3D 3
z =3D 4
`seq` has been "unpacked", and its elements have been assigned to
variables (namely: `w`, `x`, `y`, and `z`). If the number of variables
doesn't match the number of elements, Python will raise an exception.

> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D
> dataset =3D cursor.fetchall()
>
> for host, hits, agent, date in dataset:

for-loops perform repeated assignments to the loop variable(s), and,
like with the simple assignment statement example I gave, also permit
the use of unpacking in such assignments. To make the unpacking more
explicit, the loop can be equivalently rewritten as:
for _row in dataset:
    host, hits, agent, date =3D _row
    # rest same as before...

> =C2=A0 =C2=A0print ( "<tr>" )
>
> =C2=A0 =C2=A0for item in host, hits, agent, date:

Python's syntax for tuples is based solely on commas and does not
require parentheses, though a tuple's repr() always includes the
parentheses and programmers often/typically do too. (See
http://docs.python.org/tutorial/datastructures.html#tuples-and-sequences
.) For example:
x =3D 1, 2
And:
x =3D (1, 2)
Both do exactly the same thing: set `x` to a tuple of length 2
containing the elements 1 and 2.
The loop thus might be more clearly written as:
    for item in (host, hits, agent, date):
So, what's happening is that Python is iterating over the elements of
an anonymous literal tuple; `item` will thus take on the values of
`host`, `hits`, `agent`, and `date`, in turn.

> =C2=A0 =C2=A0 =C2=A0 =C2=A0print ( "<td><b><font color=3Dwhite> %s </td>"=
 % item )
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D

Cheers,
Chris
--
http://rebertia.com