[Tutor] looping through and comparing lists of dictionaries

[bkd]

I have two lists of dictionaries. One is freshly downloaded from a telnet
screenscrape, the other is read from a file that's a snapshot of the last
screenscrape:

olddata=[{'name':'Betty','hair':'Blonde','role':'Student'},
{'name':'Veronica','hair':'Brunette','role':'Student'},
{'name':'Maryann','hair':'Brunette','role':'Castaway'},
{'name':'Ginger','hair':'Redhead','role':'Castaway'},
{'name':'Jeanne','hair':'Blond','role':'Genie'},
{'name':'Serena','hair':'Brunette','role':'Genie'},
{'name':'Samantha','hair':'Blond','role':'Witch'}]

newdata=[{'name':'Betty','hair':'Redhead','role':'Student'},
{'name':'Veronica','hair':'Redhead','role':'Student'},
{'name':'Maryann','hair':'Brunette','role':'Castaway'},
{'name':'Ginger','hair':'Redhead','role':'Castaway'},
{'name':'Jeanne','hair':'Blond','role':'Genie'},
{'name':'Serena','hair':'Brunette','role':'Genie'},
{'name':'Samantha','hair':'Redhead','role':'Witch'}]

I want to find all the girls who have dyed their hair red since the last
time I checked. I start by running a loop that pulls all the names of all
the Redheads into a list:

redheads=[]
for girl in range(len(newdata)):
    if newdata[girl]['hair']=='Redhead':
redheads.append(newdata[girl]['name'])

this gives me:
['Betty','Veronica','Ginger','Samantha']
for redheads.

Now, I need to check olddata to see if any of these girls were redheads, and
eliminate them from the list.

What I have now is this:

for girl in range(len(redheads)):
    if len(redheads)==0: break
    else:
        for line in range(len(olddata)):
            if len(redheads)==0: break
            elif olddata[line]['name']<>redheads[girl]: pass
            elif olddata[line]['hair']=='Redhead': del redheads[girl]

But this breaks with an 'index out of range' error. The lesson being, do not
mess with the list you're indexing over, make a copy and mess with the copy.

But still, I can't help but think there must be a better way to do this. Any
suggestions where I should look first? Learning Python and the online
documentation seem fairly anemic when it comes to showing how to really
wield dictionaries with power, and the Python Cookbook seems to assume one
has a bit more knowledge about the language than the bundled docs provide...

bkd