A = {'a':10, 'b':843, 'c': 39,.....}

max(A.iteritems(), key=operator.itemgetter(1))[0:]

newA = dict(sorted(A.iteritems(), key=operator.itemgetter(1), reverse=True)[:5])

max(A, key=A.get)

sorted(A, key=A.get, reverse=True)[:5]

import heapq
heapq.nlargest(5, A, key=A.get)

>>> from collections import Counter
>>> A = {'a' : 1, 'b' : 3, 'c' : 2, 'd' : 4, 'e' : 0, 'f' :5}
>>> dict(Counter(A).most_common(5))
{'a': 1, 'c': 2, 'b': 3, 'd': 4, 'f': 5}

dict(sorted(A.iteritems(), key=operator.itemgetter(1), reverse=True)[:5])

import operator
dict(sorted(A.items(), key=operator.itemgetter(1), reverse=True)[:5])