Python list to dict example sharing

Requirements: [' 1: a ', '2: b', '3: c] into {' 1', 'a', '3', 'c', '2', ' '}

a = {}
b = ['1:a','2:b','3:c']
map(lambda x:a.setdefault(x.split(':')[0], x.split(':')[1]), b)
print a
{'1': 'a', '3': 'c', '2': 'b'}