On 08/07/10 01:45, Νικόλαος Κούρας wrote:
# variant B
for row in dataset:
host, hits, dt = row
# rest of your code here
So, row is a tuple comprising of 3 fields, and host, hist, dt
are variables assigned each one of row's tuple values by
breaking it to it's elements.
But what kind of objects is host, hits, dt that containes the
row's tuple data themselves? tuples or lists and why?
They contain the data of each respective element. E.g.:
>>> dataset = [
... (1, 'a', True),
... (2, 'b', False),
... ]
>>>
>>> for one, two, three in dataset:
... print 'one%s = %r' % (type(one), one)
... print 'two%s = %r' % (type(two), two)
... print 'three%s = %r' % (type(three), three)
... print '-' * 10
...
one<type 'int'> = 1
two<type 'str'> = 'a'
three<type 'bool'> = True
----------
one<type 'int'> = 2
two<type 'str'> = 'b'
three<type 'bool'> = False
----------
So likely in your case, "host" is a string, "hits" is an int, and
"dt" is a datetime.datetime object. The three of them together
are the row as represented as a tuple:
>>> type( (host, hits, dt) )
<type 'tuple'>
which you can see in your own code by changing it temporarily to:
for row in dataset:
print type(row), len(row)
# variant C
for host, hits, dt in row:
# rest of your code here
host, hits, data each and every one of them hold a piece of the row's
tuple values.
But what happens in here?
The same as Variant B, only it doesn't use the intermediate tuple
"row".
'for host, hits, dt in dataset:'
Here we don't have the row tuple. So what tthose variabels store, and in
what datatype they strore info in and what is the difference between this
and
'for host, hits, dt in row:'
The second one will fail because it would be the same as
for tpl in row:
host, hits, dt = tpl
The 1st time through the loop, tpl=host; the 2nd time through the
loop, tpl=hits; and the 3rd time through the loop, tpl=dt
Attempting to do a tuple assignment (that 2nd line) will attempt
to do something like
host, hits, dt = "example.com" # 1st pass through the loop
host, hits, dt = 42 # 2nd pass through the loop
host, hits, dt = datetime(2010,7,5)# 3rd pass through the loop
In most cases, it will fail on the first pass through the loop
(except in the freak case your string value happens to have 3
characters:
>>> host, hits, dt = "abc" #exactly 3 letters
>>> host
'a'
)
If the fieds datatypes returned form the database are for exmaple page
varchar(50) , hits inteeger[11], date datetime then
the when python grabs those results fields it would translate them to
'page as string' , (hits as int) , 'date as string' respectively?
Whcih emans it translated those fileds returned to the most
appropriate-most close to prototype stored in database' datatypes
automatically?
Yes, except the internals (of the DB module...in this case mysql)
are smart enough to translate the date into a datetime.datetime
object, instead of a string.
row = (host, hits, dt
Would that be a row or a tuple when joined?
A "row" is a conceptual thing -- one row of data from your query.
It can be represented as either a tuple or a list (or any
iteratable that represents "things in this row"). In this case
(and I believe each row returned by a cursor.fetch*() call), it
was tuple.
I hope that helps...it would behoove you to experiment with
tuple-assignments such as the example code above so that you
understand what it's doing in each case.
-tkc
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