what's going on here?
johnjsal at NOSPAMgmail.com
Thu Mar 16 17:31:38 CET 2006
This might be confusing to explain, because it's a question about an
example in Beginning Python and I'll try to provide all the info I can.
First off, I'm reading a chapter on using the ReportLab modules to
create a line graph from a set of data. The first implementation of the
program uses a hard-coded list as the data source, the second
implementation pulls the data from a URL. In either case, the data is of
# year month predicted high low
2004 12 34.2 35.2 33.2
2005 01 31.5 34.5 28.5
(repeated many times)
In the first implementation, the data was a list of tuples, each tuple
being one row of the data. So grabbing the data was done like this:
pred = [row-40 for row in data]
high = [row-40 for row in data]
We can safely ignore the '-40', that was just for positioning. So the
variable for predicted would grab 34.2, 31.5, etc., high would get 35.2,
etc. Easy enough.
Now, the second implementation does this:
for line in urlopen(URL).readlines():
if not line.isspace() and not line in COMMENT_CHARS:
pred = [row for row in data]
high = [row for row in data]
(URL is the location of the data file online. The if statement just
checks for blank lines and lines beginning with certain comment
characters, so they can be ignored.)
So finally here's my question: If you are using data.append(), doesn't
that just put all the numbers into one long list? How are the tuples
still being created in this case so that the list comprehensions still
work? It seems like there is no longer any 'row' to refer to in data.
The only thing I can think of is that each time through the for loop, a
new item (tuple or list) is being created in the data list, so that each
row of data really is being separated as its own element in the larger
list, but that doesn't seem right.
More information about the Python-list