Converting list of lists into a dictionary of dictionaries in Python

Question

I am trying to convert a list of lists data structure to a dictionary of dictionaries.

The list is defined as follows:

l = [   ['PP','Ear-rings', 'Holesovice', 2000],   ['PP','Skirts', 'Holesovice', 1000],   ['PP','Dresses', 'E-shop', 1500],   ['BM','Butterfly', 'Holesovice', 1600] ] 

My aim is to have the dictionary structure as follows:

#{'PP' : {'Holesovice' : {'Ear-rings' : 2000, 'Skirts' : 1000}, #         'E-shop' : {'Dresses' : 1500}}, # 'BM' : {'Holesovice' : {'Butterfly' : 1600}} #} 

This bit of code does not return desired output:

labels_d = {} items_d = {} shops_d = {}  for index, row in enumerate(l):   items_d[row[1]] = row[3]   shops_d[row[2]] = items_d   labels_d[row[0]] = shops_d  print(labels_d) 

I found some posts that deal with converting lists to dictionaries here and here but I did not make it work the way I want. Is there any 'clean' way how to achieve the structure posted above?

Answer 1

Using dict.setdefault(key, {}) is a good way to approach the creation of nested dictionaries of fixed depth.

l = [   ['PP','Ear-rings', 'Holesovice', 2000],   ['PP','Skirts', 'Holesovice', 1000],   ['PP','Dresses', 'E-shop', 1500],   ['BM','Butterfly', 'Holesovice', 1600] ]  d = {}  for tag, item, source, qty in l:     d.setdefault(tag, {}).setdefault(source, {})[item] = qty  

Output

{'BM': {'Holesovice': {'Butterfly': 1600}},  'PP': {'E-shop': {'Dresses': 1500},         'Holesovice': {'Ear-rings': 2000, 'Skirts': 1000}}} 

Generalization

The above solution can be made more general by building a class of nested dictionary, dropping the requirements to have a fixed depth.

class NestedDict(dict):     def __getitem__(self, item):         if item not in self:             self[item] = NestedDict()         return super().__getitem__(item)  d = NestedDict()  for tag, item, source, qty in l:     d[tag][source][item] = qty  

Also notice that the class approach is created so it only creates an object if the key does not exist while the setdefault approach created an empty dict on every access.