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I have a large list like:
[A][B1][C1]=1 [A][B1][C2]=2 [A][B2]=3 [D][E][F][G]=4 I want to build a multi-level dict like:
A --B1 -----C1=1 -----C2=1 --B2=3 D --E ----F ------G=4 I know that if I use recursive defaultdict I can write table[A][B1][C1]=1, table[A][B2]=2, but this works only if I hardcode those insert statement.
While parsing the list, I don't how many []'s I need beforehand to call table[key1][key2][...].
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Finally, using a defaultdict to handle missing keys can be faster than using dict.
Now that we understand nested dictionaries and defaultdicts , we can get into nested defaultdicts . This is concept is extremely powerful as it allows you to build complex dictionaries with a simple initialization. The only caveat is that you need to know the depth of your data structure in advance.
The difference is that a defaultdict will "default" a value if that key has not been set yet. If you didn't use a defaultdict you'd have to check to see if that key exists, and if it doesn't, set it to what you want. The lambda is defining a factory for the default value.
defaultdict takes a zero-argument callable to its constructor, which is called when the key is not found, as you correctly explained. lambda: 0 will of course always return zero, but the preferred method to do that is defaultdict(int) , which will do the same thing.
You can do it without even defining a class:
from collections import defaultdict nested_dict = lambda: defaultdict(nested_dict) nest = nested_dict() nest[0][1][2][3][4][5] = 6 Your example says that at any level there can be a value, and also a dictionary of sub-elements. That is called a tree, and there are many implementations available for them. This is one:
from collections import defaultdict class Tree(defaultdict): def __init__(self, value=None): super(Tree, self).__init__(Tree) self.value = value root = Tree() root.value = 1 root['a']['b'].value = 3 print root.value print root['a']['b'].value print root['c']['d']['f'].value Outputs:
1 3 None You could do something similar by writing the input in JSON and using json.load to read it as a structure of nested dictionaries.
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