new_dic[1][2] = 5
KeyError: 1

new_dic = {}
new_dic[1] = {}
new_dic[1][2] = 5

new_dic.setdefault(1, {})[2] = 5

from collections import defaultdict

new_dic = defaultdict(dict)
new_dic[1][2] = 5

>>> new_dic_plain = {}
>>> new_dic_plain[1] = {}
>>> new_dic_plain[1][2] = 5
>>> new_dic_plain
{1: {2: 5}}
>>> new_dic_setdefault = {}
>>> new_dic_setdefault.setdefault(1, {})[2] = 5
>>> new_dic_setdefault
{1: {2: 5}}
>>> from collections import defaultdict
>>> new_dic_defaultdict = defaultdict(dict)
>>> new_dic_defaultdict[1][2] = 5
>>> new_dic_defaultdict
defaultdict(<type 'dict'>, {1: {2: 5}})

def nested_dict(n, type):
    if n == 1:
        return defaultdict(type)
    else:
        return defaultdict(lambda: nested_dict(n-1, type))

new_dict = nested_dict(2, float)

new_dict['key1']['key2'] += 5

new_dict = nested_dict(3, list)
new_dict['a']['b']['c'].append(5)

new_dict['a']['b']['c'] = [5]

new_dic = {}
try:
    new_dic[1][2] = 5
except KeyError:
    new_dic[1] = {2:5}

>>> new_dic = {}
>>> new_dic[1] = {2:5}
>>> new_dic
{1: {2: 5}}

new_dic={}
new_dic[1][2]=5

from collections import defaultdict
new_dic = defaultdict(dict)
new_dic[1][2]=5
>>>new_dic
defaultdict(<type 'dict'>, {1: {2: 5}})

new_dic = {}
a = 1
new_dic[a] = {}
b = 2
new_dic[a][b] = {}
c = 5
new_dic[a][b]={c}

new_dic[a][b]
>>>'5'

new_dic = {}
a = "cat"
new_dic[a] = {}
b = "dog"
new_dic[a][b] = {}
c = 5
new_dic[a][b] = {c}

new_dic["cat"]["dog"]
>>>'5'