Explanation about for

[Νικόλαος Κούρας]

Thank you All for the detailed examples.

I tried them all in IDLE and i finally understood them.

Thanks for your patience with me until i understand!

2012/1/10 Nick Dokos <nicholas.dokos at hp.com>

> Νικόλαος Κούρας <nikos.kouras at gmail.com> wrote:
>
> > On 10 Ιαν, 03:11, Ian Kelly <ian.g.ke... at gmail.com> wrote:
> > > 2012/1/9 Íéêüëáïò Êïýñáò <nikos.kou... at gmail.com>:
> > >
> > > > if the MySQL query was:
> > >
> > > > cursor.execute( '''SELECT host, hits, agent, date FROM visitors
> WHERE pin =
> > > > %s ORDER BY date DESC''', pin )
> > >
> > > > can you help me imagine how the mysql database cursor that holds the
> query
> > > > results would look like? I must somehow visualize it in order to
> understand
> > > > it!
> > >
> > > You can think of it as a pointer, traversing over one row of the
> > > result set at a time.  Hopefully this will come out legibly:
> > >
> > > -----------------------------------------------
> > > | HOST | HITS | AGENT | DATE |
> > > -----------------------------------------------
>  -------------
> > > | foo     | 7       | IE6       | 1/1/11 |   <----   | cursor |
> > > -----------------------------------------------
>  -------------
> > > | bar     | 42     | Firefox  | 2/2/10 |
> > > -----------------------------------------------
> > > | baz    | 4       | Chrome | 3/3/09 |
> > > ------------------------------------------------
> >
> > Database cursor is the pointer that iterates over the result set one
> > row at a time?
> > I though that it was the name we give to the whole mysql result set
> > returned my cursor.execute.
> >
> > >
> > >
> > > > Also what happend if the query was:
> > > > cursor.execute( '''SELECT host FROM visitors") ?
> > >
> > > > the result would have to be something likelike?
> > >
> > > > -----------------
> > > > |somehost1|
> > > > -----------------
> > > > |somehost2|
> > > > -----------------
> > > > |somehost3|
> > > > -----------------
> > > > .....................
> > > > .....................
> > > > |somehost n|
> > > > -----------------
> > >
> > > > So what values host, hits, agent, date would have in 'for host,
> hits, agent,
> > > > date in
> > > >  dataset' ? Every row has one string how can that be split in 4?
> > >
> > > Why don't you try it and see what happens?  But to spare you the
> > > suspense, you would get:
> > >
> > > ValueError: need more than 1 value to unpack
> > >
> > > Because you can't unpack a 1-length tuple into four variables.  The
> > > code assumes that the query is selecting exactly 4 columns.
> >
> >
> > -----------------------------------------------
> > | HOST    | HITS    | AGENT     | DATE |
> > -----------------------------------------------
> > | foo     | 7       | IE6       | 1/1/11 |
> > -----------------------------------------------
> > | bar     | 42      | Firefox   | 2/2/10 |
> > -----------------------------------------------
> > | baz     | 4       | Chrome    | 3/3/09 |
> > -----------------------------------------------
> >
> > In this line:
> > for host, hits, agent, date in dataset:
> >
> > 'dataset' is one of the rows of the mysql result or the whole mysql
> > result set like the table above?
> >
> > I still have trouble understanding this line :(
>
> You can think of it as a list of tuples. Forget about cursors and
> databases for now. If l is a list [1, 2, 3, 4] you iterate over it like
> this:
>
> for x in l:
>  print x
>
> and you get each element of the list[fn:1]. Similarly if l is a list of
> tuples
> l = [(1, 2, 3, 4), (5, 6, 7, 8), (9, 10, 11, 12)] you can iterate over the
> list:
>
> for x in l:
>  print x
>
> In this case, x is going to be (1,2,3,4) the first time through the loop,
> (5,6,7,8)
> the second time and so on. You can break each x apart within the loop:
>
> for x in l:
>  t1, t2, t3, t4 = x
>  print x, t1, t2, t3, t4
>
> or you can break it apart like this - it's essentially the same thing:
>
> for t1, t2, t3, t4 in l:
>  print t1, t2, t3, t4
>
> You have been encouraged repeatedly to try these things interactively:
> please do so with the above examples and all will become clear.
>
> Going back to cursors and databases: you *can* think of 'dataset' as
> being a list of tuples - a list of all the query results, but with one
> proviso. The difference when you use a cursor is that `dataset' may
> be a lazy list (an "iterator"): instead of the whole set of results
> being in memory at the same time, the system will take care of doing
> whatever is necessary to get more of the results when it needs them. But
> the behavior is the *same* in the sense that the output that you get is
> the same (you will only see differences if you are keeping an eye on how
> much memory and how much time your program is using).
>
> Nick
>
> Footnotes:
>
> [fn:1] ... and, no, you *can't express* this as
> "the first time it is
>
> for x in 1:
>  ...
>
> and the second time it is
>
> for x in 2:
>  ...
>
> " as you asked in another email. That's arrant nonsense: x takes
> successive values in the list l for every iteration of the for
> loop. This is elementary Python (nay, elementary programming, period).
>
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