>>> h = {'a' : 1}
>>> h['a'] += 2
>>> h
{'a': 3}
>>>
>>> from collections import OrderedDict
>>> oh = OrderedDict(h)
>>> oh
OrderedDict([('a', 3)])
>>> oh['a'] += 2
>>> oh
OrderedDict([('a', 5)])
>>> oh['b'] = 'hello'
>>> oh['b'] += ' world'
>>> oh
OrderedDict([('a', 5), ('b', 'hello world')])