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.do...@hp.com> > Νικόλαος Κούρας <nikos.kou...@gmail.com> wrote: > > > On 10 Ιαν, 03:11, Ian Kelly <ian.g.ke...@gmail.com> wrote: > > > 2012/1/9 Íéêüëáïò Êïýñáò <nikos.kou...@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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