Get length of dictionary in Python

To calculate the length of a dictionary, we can use the Python built-in len() method. The len()  method returns the number of keys in a Python dictionary.

Python Dict len() Syntax

Syntax: len(Dict)

Return: It returns an integer which is the length of the string. 

Name:Steve
Age:30
Designation:Programmer

Basic example of finding the length of a dictionary

Output:

Dictionary: {'Name': 'Steve', 'Age': 30, 'Designation': 'Programmer'}
Length of dictionary: 3

Finding length of nested dictionary

Consider the following details about a person:

Name:Steve
Age:30
Designation:Programmer
address:
      Street:Brigade  Road
      City:Bangalore
      Country:India

Problem when trying to find length of nested dictionary:

Output:
len() method with keys() : 4
len() method with values(): 4

Here, whichever method you apply, You will get only ‘4’ as the output. But the actual number of entries is ‘7’. The keys are name, age, designation, address, street, city, and country. The method considers the outer dictionary which is a value for one of the keys as a single value.

To overcome this problem, we need to explicitly add the length of the inner dictionary to the outer one. It can be coded as given below: 

Output:

The length of the nested dictionary is: 7

Now it works fine!!! However, is it possible to explicitly program to add the length of inner dictionaries every time? What if we do not know in prior how many inner dictionaries are there? Now consider the following detail:

Name:
    first name:Steve
    last name:Jobs
Age:30
Designation:Programmer
address:
      Street:Brigade  Road
      City:Bangalore
      Country:India

Here we have two inner dictionaries. It is not an optimized way to explicitly add the length of the inner dictionaries every time. We can solve this problem by combining isinstance() with len() method. The idea is to first store the length of the whole dictionary in a variable (say ‘length’ here). Then iterate through all the values()of the dictionary and check whether it is an instance of dict. If ‘True’ then the length of that inner dictionary is found and added to the variable length. In this way, the total length of a nested dictionary could be found.

Example 1: Finding length of nested dictionary dynamically using for-loop:

When we have more keys in a dictionary whose values are again dictionaries. Then we need to check if the type of the value of each key, if it’s a dictionary, then we use len() on the value and add the value to the length of the outer dictionary.

Output:

The length of the dictionary is  9

Note: This approach will only work when the nesting of the dictionaries is only upto 2 levels.

If the dictionary is further deeply nested like below:

Name:
    first name:Steve
    last name:Jobs
Age:30
Designation:Programmer
address:
      Street:
          St_number:4
          St_name:Brigade  Road
      City:Bangalore
      Country:India

Example 2: Using Recursion to find length of nested dictionary:

Here we have used a recursive function count_nested_len() to count the length of each dictionary, we iterate on the keys of dictionaries, as soon as a value is a dictionary, we recuse on that dictionary.

Output:

Nested dictionary length: 11

Approach 3 : Using Dictionary Comprehension

Python3

# nested dictionary
dict2 ={
      ‘Name’:
          {
   ‘first_name’:’Steve’,
              ‘Last_name’:’Jobs’
          },
      ‘Age’:30,
      ‘Designation’:’Programmer’,
      ‘address’:
          {
          ‘Street’:
              {
                  ‘st_number’:4,
                  ‘st_name’:’Rockins Road’
              },
          ‘City’:’Bangalore’,
          ‘Country’:’India’
          }      
     }

# Using dictionary comprehension to find the length of the nested dictionary
length = len({k: v for k, v in dict2.items()})

print(“The length of the dictionary is”, length)
#This code is contributed by Edula Vinay Kumar Reddy
 

METHOD 4:Using a generator expression with sum() function.

APPROACH:

A generator expression is used to create a sequence of 1’s, where each 1 corresponds to a key in the dictionary. The sum() function is then used to add up the 1’s, which gives the length of the dictionary.

ALGORITHM:

1.Create a generator expression that generates a sequence of 1’s for each key in the dictionary.
2.Use the sum() function to add up the 1’s in the sequence.
3.Return the sum, which represents the length of the dictionary.

3

The time complexity of this approach is O(n), where n is the number of keys in the dictionary.

The space complexity of this approach is O(1)