[Tutor] basic lists and loops question

[bob gailer]

Jon Crump wrote:
> Something basic about lists and loops that I'm not getting here. I've 
> got a long list of dictionaries that looks like this:
>
> lst = [{'placename': u'Stow, Lincolnshire', 'long-name': u'Stow, 
> Lincolnshire.', 'end': datetime.date(1216, 9, 28), 'start': 
> datetime.date(1216, 9, 26)},
> {'placename': u'Lincoln, Lincolnshire', 'long-name': u'Lincoln, 
> Lincolnshire.', 'end': datetime.date(1216, 9, 30), 'start': 
> datetime.date(1216, 9, 28)},
> {'placename': u'Lincoln, Lincolnshire', 'long-name': u'Lincoln, 
> Lincolnshire.', 'end': datetime.date(1216, 10, 2), 'start': 
> datetime.date(1216, 10, 1)},
> {'placename': u'Grimsby, Lincolnshire', 'long-name': u'Grimsby, 
> Lincolnshire.', 'end': datetime.date(1216, 10, 4), 'start': 
> datetime.date(1216, 10, 3)},
> {'placename': u'Louth, Lincolnshire', 'long-name': u'Louth, 
> Lincolnshire.', 'end': datetime.date(1216, 10, 4), 'start': 
> datetime.date(1216, 10, 4)}
> ]
>
> I have a function that searches through them to find pairs of 
> dictionaries that satisfy certain criteria. When the nested loops find 
> such a pair, I need to merge them. So far so good. This works:
>
> def events(data):
>   evts = []
>   for x in lst:
>     for y in lst:
>       if (x['placename'] == y['placename']) and (x['end'].month + 1 == 
> y['start'].month) and (y['start'] - x['end'] == datetime.timedelta(1)):
>         evts.append({'placename': x['placename'], 'long-name': 
> x['long-name'], 'start': x['start'], 'end': y['end']})
>     evts.append(x)
>   return evts
>
> for x in events(lst):
>   print x
>
> But then I need to delete the two original dictionaries that I merged. 
> If I do del x, I get an error "local variable 'x' referenced before 
> assignment"

Try lst.remove(x)
>
> I've also tried decorating the processed dictionaries in the if loop 
> thus:
> x['processed'] = True
> y['processed'] = True
>
> Then when I call events() I get back the merged dict and the decorated 
> dicts:
>
> {'placename': u'Lincoln, Lincolnshire', 'end': datetime.date(1216, 10, 
> 2), 'start': datetime.date(1216, 9, 28), 'long-name': u'Lincoln, 
> Lincolnshire.'}
> {'placename': u'Lincoln, Lincolnshire', 'processed': True, 'end': 
> datetime.date(1216, 9, 30), 'start': datetime.date(1216, 9, 28), 
> 'long-name': u'Lincoln, Lincolnshire.'}
> {'placename': u'Lincoln, Lincolnshire', 'processed': True, 'end': 
> datetime.date(1216, 10, 2), 'start': datetime.date(1216, 10, 1), 
> 'long-name': u'Lincoln, Lincolnshire.'}
>
> But if I try to call events() thus:
>
> for x in events(lst):
>   if x['processed'] == True:
>     print x
>
> I get a KeyError.

Sounds like the key 'processed' is created by the assignment 
x['processed'] = True. So those dictionaries that have not experienced 
this assignment have no such key. You should instead use: if  
'processed' in x:

Also if x['processed'] == True: can be expressed if x['processed']: 
>  

HTH

-- 
Bob Gailer
919-636-4239 Chapel Hill, NC