list = [{'key':1, 'a':100, 'b':50, 'c':80},
        {'key':2, 'a':200, 'b':60, 'c':70},
        {'key':2, 'a':300, 'b':70, 'c':60},
        {'key':3, 'a':400, 'b':80, 'c':50},
        {'key':4, 'a':500, 'b':90, 'c':40},
        {'key':4, 'a':600, 'b':40, 'c':30}]

list = [{'key':1, 'a':100, 'b':50, 'c':80},
        {'key':2, 'a':500, 'b':130, 'c':130},
        {'key':3, 'a':400, 'b':80, 'c':50},
        {'key':4, 'a':1100, 'b':130, 'c':70}]

list = [
 {'key':1, 'a':100, 'b':50, 'c':80},
 {'key':2, 'a':200, 'b':60, 'c':70},
 {'key':2, 'a':300, 'b':70, 'c':60},
 {'key':3, 'a':400, 'b':80, 'c':50},
 {'key':4, 'a':500, 'b':90, 'c':40},
 {'key':4, 'a':600, 'b':40, 'c':30}]
 
conn = sqlite3.connect(":memory:")

conn.execute("create table list (key, a, b)")
conn.executemany("insert into list (key, a, b) values (:key, :a, :b)",list)
cur = conn.execute("select key, SUM(a), SUM(b) from list group by key")
print(cur.fetchall())

lis = [{'key':1, 'a':100, 'b':50, 'c':80},
        {'key':2, 'a':200, 'b':60, 'c':70},
        {'key':2, 'a':300, 'b':70, 'c':60},
        {'key':3, 'a':400, 'b':80, 'c':50},
        {'key':4, 'a':500, 'b':90, 'c':40},
        {'key':4, 'a':600, 'b':40, 'c':30}]

from itertools import groupby

res = []
for k,l in groupby(lis, lambda x:x['key']):
    l = list(l)
    if len(l) > 1:
        l = dict( [ reduce(lambda a,b: ( a[0],a[1]+b[1]) , x) for x in map(None, *[x.items() for x in l]) ] )
        l.update({ 'key':k })
    else: l = l[0]
    res.append(l)

print res

li = [{'key':1, 'a':100, 'b':50, 'c':80},
        {'key':2, 'a':200, 'b':60, 'c':70},
        {'key':2, 'a':300, 'b':70, 'c':60},
        {'key':3, 'a':400, 'b':80, 'c':50},
        {'key':4, 'a':500, 'b':90, 'c':40},
        {'key':4, 'a':600, 'b':40, 'c':30}]

listOfDict=[[rec.pop('key'),rec] for rec in li]

def dictadd(a,b):
    for k,v in b.iteritems():
        vv=a.get(k,0)
        a[k]=vv+v

res={}
for key,val in listOfDict:
    vv=res.get(key,{})
    dictadd(vv,val)
    res[key]=vv