Sometimes you may need to have nested dictionaries initialized without much hassle.
“Autovivication” comes in handy. What you want to achieve is:
>>> d = dict()
>>> d['q']['w']['r'] = 777
Of course what you’ll get will be:
>>> d['q']['w']['r'] = 777
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'q'
If you don’t need deeply nested dictionaries defaultdict with dict default factory will do the job:
>>> from collections import defaultdict
>>> d = defaultdict(dict)
>>> d
defaultdict(<type 'dict'>, {})
>>> d['a']['b'] = 33
>>> d
defaultdict(<type 'dict'>, {'a': {'b': 33}})
>>> d['a']['b']
33
Here are another ways to have unlimited level of nested dictionaries.
1) Define custom class inheriting from built-in dict:
class default_dict(dict):
def __missing__(self, key):
self[key] = value = self.__class__()
return value
>>> d = default_dict()
>>> d
{}
>>> d['q']['w']['r'] = 777
>>> d
{'q': {'w': {'r': 777}}}
>>> d['q']['w']['r']
777
2) Recursive function + defaultdict:
def make_dict():
return defaultdict(make_dict)
>>> from collections import defaultdict
>>> d = defaultdict(make_dict)
>>> d
defaultdict(<function make_dict at 0xb7c5295c>, {})
>>> d['q']['w']['r'] = 777
>>> d
defaultdict(<function make_dict at 0xb7c5295c>, {'q': defaultdict(<function make_dict at 0xb7c5295c>, {'w': defaultdict(<function make_dict at 0xb7c5295c>, {'r': 777})})})
>>> d['q']['w']['r']
777
3) And for fun you can use Y Combinator (in short – making anonymous lambda recursive) which is actually as (2) but you don’t have to have recursive function defined:
def Y(g):
return ((lambda f: f(f))
(lambda f:
g(lambda x: f(f)
(x))))
defdict = Y(lambda mk_dict:
lambda x=None: defaultdict(lambda x=None: mk_dict(x)))
>>> d = defdict()
>>> d
defaultdict(<function <lambda> at 0xb7c52a74>, {})
>>> d['q']['w']['r'] = 777
>>> d
defaultdict(<function <lambda> at 0xb7c52a74>, {'q': defaultdict(<function <lambda> at 0xb7c56c6c>, {'w': defaultdict(<function <lambda> at 0xb7c56a74>, {'r': 777})})})
>>> d['q']['w']['r']
777
Posted by Ruslan Spivak 
Posted by Ruslan Spivak 
Posted by Ruslan Spivak 