default_dict_eg.py

from collections import Counter, defaultdict

food = [{'date':'2014', 'address':'sesame street'},{'date':'2012', 'address':'eversholt street'},{'date':'2013', 'address':'eversholt street'},{'date':'2014', 'address':'eversholt street'},{'date':'2012', 'address':'sesame street'},{'date':'2013', 'address':'sesame street'},{'date':'2014', 'address':'sesame street'},{'date':'2014', 'address':'eversholt street'},{'date':'2014', 'address':'sesame street'}]

def short_way(food):
	## a default dict sets the value type to the 
	by_year = defaultdict(Counter)
	for row in food:
		# print by_year
		## simultaneously adds the date as a key to the dict and incremements the counter value for the address to += 1
		by_year[row['date']][row['address']] += 1
	print by_year

def long_way(food):
	by_year = {}
	for row in food:
		if row['date'] in by_year:
			by_year[row['date']].append(row['address'])
		else:
			by_year[row['date']] = []
			by_year[row['date']].append(row['address'])
	year_summary = {}
	for year in by_year:
		year_summary[year] = Counter(by_year[year])
	print year_summary

short_way(food)
# long_way(food)