Building Nested dictionary in Python reading in line by line from file

Question

The way I go about nested dictionary is this:

dicty = dict()
tmp = dict()
tmp[\"a\"] = 1
tmp[\"b\"] = 2
dicty[\"A\"] = tmp

dicty == {\"A\" : {\"a\" : 1, \"b         


        

Answer 1

Use a collections.defaultdict() object to auto-instantiate nested dictionaries:

from collections import defaultdict

def doubleDict(filename):
    dicty = defaultdict(dict)
    with open(filename, "r") as f:
        for i, line in enumerate(f):
            outer, inner, value = line.split()
            dicty[outer][inner] = value
            if i % 25 == 0:
                print(dicty)
                break #print(row)
    return(dicty)

I used enumerate() to generate the line count here; much simpler than keeping a separate counter going.

Even without a defaultdict, you can let the outer dictionary keep the reference to the nested dictionary, and retrieve it again by using values[0]; there is no need to keep the temp reference around:

>>> dicty = {}
>>> dicty['A'] = {}
>>> dicty['A']['a'] = 1
>>> dicty['A']['b'] = 2
>>> dicty
{'A': {'a': 1, 'b': 1}}

All the defaultdict then does is keep us from having to test if we already created that nested dictionary. Instead of:

if outer not in dicty:
    dicty[outer] = {}
dicty[outer][inner] = value

we simply omit the if test as defaultdict will create a new dictionary for us if the key was not yet present.