Cleaning up conditionals

[Deborah Swanson]

Peter Otten wrote:
> Deborah Swanson wrote:
> 
> > Here I have a real mess, in my opinion:
> 
> [corrected code:]
> 
> >         if len(l1[st]) == 0:
> >             if len(l2[st]) > 0:
> >                 l1[st] = l2[st]
> >         elif len(l2[st]) == 0:
> >             if len(l1[st]) > 0:
> >                 l2[st] = l1[st]
> 
> > Anybody know or see an easier (more pythonic) way to do 
> this? I need 
> > to do it for four fields, and needless to say, that's a really long 
> > block of ugly code.
> 
> By "four fields", do you mean four values of st, or four 
> pairs of l1, l2, or 
> more elif-s with l3 and l4 -- or something else entirely?
> 
> Usually the most obvious way to avoid repetition is to write 
> a function, and 
> to make the best suggestion a bit more context is necessary.
> 

I did write a function for this, and welcome any suggestions for
improvement.

The context is comparing 2 adjacent rows of data (in a list of real
estate listings sorted by their webpage titles and dates) with the
assumption that if the webpage titles are the same, they're listings for
the same property. This assumption is occasionally bad, but in far less
than one per 1000 unique listings. I'd rather just hand edit the data in
those cases so one webpage title is slightly different, than writing and
executing all the code needed to find and handle these corner cases.
Maybe that will be a future refinement, but right now I don't really
need it.

Once two rows of listing data have been identified as different dates
for the same property, there are 4 fields that will be identical for
both rows. There can be up to 10 (or even more) listings identical
except for the date, but typically I'm just adding a new one and want to
copy the field data from its previous siblings, so the copying is just
from the last listing to the new one.

Here's the function I have so far:

def comprows(l1,l2,st,ki,no):
    ret = ''
    labels = {st: 'st/co', ki: 'kind', no: 'notes'}
    for v in (st,ki,no):
        if len(l1[v]) == 0 and len(l2[v]) != 0:
            l1[v] = l2[v]
        elif len(l2[v]) == 0 and len(l1[v]) != 0:
            l2[v] = l1[v]
        elif l1[v] != l2[v]:
            ret += ", " + labels[v] + " diff" if len(ret) > 0 else 
            labels[v] + " diff"
    return ret

The 4th field is a special case and easily dispatched in one line of
code before this function is called for the other 3.

l1 and l2 are the 2 adjacent rows of listing data, with st,ki,no holding
codes for state/county, kind (of property) and notes. I want the
checking and copying to go both ways because sometimes I'm backfilling
old listings that I didn't pick up in my nightly copies on their given
dates, but came across them later.

ret is returned to a field with details to look at when I save the list
to csv and open it in Excel. The noted diffs will need to be reconciled.

I tried to use Jussi Piitulainen's suggestion to chain the conditionals,
but just couldn't make it work for choosing list elements to assign to,
although the approach is perfect if you're computing a value.

Hope this is enough context... ;)
D