How do you make a nested dictionary dynamically in python?

Here's an updated version of my answer in which leaves of the tree data-structure are now different from those in rest of it. Instead of the tree being strictly a dict-of-nested-dicts, the "leaves" on each branch are now instances of a different subclass of dict named collections.Counter which are useful for counting the number of times each of their keys occur. I did this because of your response to my question about what should happen if the last part of each line was something other than ":pass" [which was "we have to put new count for that key"].

Nested dictionaries are often called Tree data-structures and can be defined recursively — the root is a dictionary as are the branches. The following uses a dict subclass instead of a plain dict because it makes constructing them easier since you don't need to special case the creation of the first branch of next level down [except I still do when adding the "leaves" because they are a different subclass, collections.Counter].

from collections import Counter
from functools import reduce
import re


# [Optional] trick to make Counter subclass print like a regular dict.
class Counter[Counter]:
    def __repr__[self]:
        return dict[self].__repr__[]


# Borrowed from answer @ //stackoverflow.com/a/19829714/355230
class Tree[dict]:
    def __missing__[self, key]:
        value = self[key] = type[self][]
        return value


# Utility functions based on answer @ //stackoverflow.com/a/14692747/355230
def nested_dict_get[nested_dict, keys]:
    return reduce[lambda d, k: d[k], keys, nested_dict]

def nested_dict_set[nested_dict, keys, value]:
    nested_dict_get[nested_dict, keys[:-1]][keys[-1]] = value

def nested_dict_update_count[nested_dict, keys]:
    counter = nested_dict_get[nested_dict, keys[:-1]]
    if counter:  # Update existing Counter.
        counter.update[[keys[-1]]]
    else:  # Create a new  Counter.
        nested_dict_set[nested_dict, keys[:-1], Counter[[keys[-1]]]]


d = Tree[]
pat = re.compile[r'[a-zA-z]+']
with open['abc.txt'] as file:
    for line in file:
        nested_dict_update_count[d, [w for w in pat.findall[line.rstrip[]]]]

print[d]  # Prints like a regular dict.

To test the leaf-counting capabilities of the revised code, I used the following test file which includes the same line twice, once ending again with :pass and another ending in :fail.

Expanded abc.txt test file:

abc/pqr/lmn/xyz:pass
abc/pqr/lmn/bcd:pass
abc/pqr/lmn/xyz:fail
abc/pqr/lmn/xyz:pass

Output:

{'abc': {'pqr': {'lmn': {'bcd': {'pass': 1}, 'xyz': {'fail': 1, 'pass': 2}}}}}

Prerequisite – Python dictionary
A Dictionary in Python works similar to the Dictionary in the real world. Keys of a Dictionary must be unique and of immutable data type such as Strings, Integers and tuples, but the key-values can be repeated and be of any type.
Nested Dictionary: Nesting Dictionary means putting a dictionary inside another dictionary. Nesting is of great use as the kind of information we can model in programs is expanded greatly. 
 

Python3

nested_dict = { 'dict1': {'key_A': 'value_A'},

                'dict2': {'key_B': 'value_B'}}

Python3

Dict = {1: 'Geeks', 2: 'For', 3: {'A' : 'Welcome', 'B' : 'To', 'C' : 'Geeks'}}

Creating a Nested Dictionary

In Python, a Nested dictionary can be created by placing the comma-separated dictionaries enclosed within braces. 
 

Python3

Dict = { 'Dict1': { },

         'Dict2': { }}

print["Nested dictionary 1-"]

print[Dict]

Dict = { 'Dict1': {'name': 'Ali', 'age': '19'},

         'Dict2': {'name': 'Bob', 'age': '25'}}

print["\nNested dictionary 2-"]

print[Dict]

Dict = { 'Dict1': {1: 'G', 2: 'F', 3: 'G'},

         'Dict2': {'Name': 'Geeks', 1: [1, 2]} }

print["\nNested dictionary 3-"]

print[Dict]

Output:
 

Nested dictionary 1-
{'Dict1': {}, 'Dict2': {}}

Nested dictionary 2-
{'Dict1': {'name': 'Ali', 'age': '19'}, 'Dict2': {'name': 'Bob', 'age': '25'}}

Nested dictionary 3-
{'Dict1': {1: 'G', 2: 'F', 3: 'G'}, 'Dict2': {1: [1, 2], 'Name': 'Geeks'}}

Adding elements to a Nested Dictionary

Addition of elements to a nested Dictionary can be done in multiple ways. One way to add a dictionary in the Nested dictionary is to add values one be one, Nested_dict[dict][key] = ‘value’. Another way is to add the whole dictionary in one go, Nested_dict[dict] = { ‘key’: ‘value’}. 
 

Python3

Dict = { }

print["Initial nested dictionary:-"]

print[Dict]

Dict['Dict1'] = {}

Dict['Dict1']['name'] = 'Bob'

Dict['Dict1']['age'] = 21

print["\nAfter adding dictionary Dict1"]

print[Dict]

Dict['Dict2'] = {'name': 'Cara', 'age': 25}

print["\nAfter adding dictionary Dict1"]

print[Dict]

Output: 
 

Initial nested dictionary:-
{}

After adding dictionary Dict1
{'Dict1': {'age': 21, 'name': 'Bob'}}

After adding dictionary Dict1
{'Dict1': {'age': 21, 'name': 'Bob'}, 'Dict2': {'age': 25, 'name': 'Cara'}}

Access elements of a Nested Dictionary

In order to access the value of any key in nested dictionary, use indexing [] syntax. 
 

Python3

Dict = { 'Dict1': {'name': 'Ali', 'age': '19'},

         'Dict2': {'name': 'Bob', 'age': '25'}}

print[Dict['Dict1']['name']]

print[Dict['Dict2']['age']]

Output: 
 

Ali
25

Deleting dictionaries from a Nested Dictionary

Deletion of dictionaries from a nested dictionary can be done either by using del keyword or by using pop[] function. 
 

Python3

Dict = {'Dict1': {'name': 'Ali', 'age': 19},

        'Dict2': {'name': 'Bob', 'age': 21}}

print["Initial nested dictionary:-"]

print[Dict]

print["\nDeleting Dict2:-"]

del Dict['Dict2']

print[Dict]

print["\nDeleting Dict1:-"]

Dict.pop['Dict1']

print [Dict]

Output: 
 

Initial nested dictionary:-
{'Dict2': {'name': 'Bob', 'age': 21}, 'Dict1': {'name': 'Ali', 'age': 19}}

Deleting Dict2:-
{'Dict1': {'name': 'Ali', 'age': 19}}

Deleting Dict1:-
{}

Are dictionaries in Python dynamic?

What is nested dictionary in Python?

Is it possible to make nested dictionary?

Is nested dictionary allowed in Python?