Python – Find occurrences for each value of a particular key

When working with a list of dictionaries, you may need to find how many times each value appears for a particular key. Python provides several approaches to count occurrences of values for a specific key.

Using collections.Counter

The most straightforward approach is using Counter to count occurrences ?

from collections import Counter

my_dict = [
    {'pyt': 13, 'fun': 44}, 
    {'pyt': 63, 'best': 15},
    {'pyt': 24, 'fun': 34}, 
    {'pyt': 47, 'best': 64},
    {'pyt': 13, 'fun': 55}  # Adding duplicate value for demonstration
]

print("The dictionary list is:")
print(my_dict)

my_key = 'pyt'
print(f"\nCounting occurrences for key: {my_key}")

# Extract values for the key and count occurrences
values = [d[my_key] for d in my_dict if my_key in d]
result = dict(Counter(values))

print("The result is:")
print(result)

The dictionary list is:
[{'pyt': 13, 'fun': 44}, {'pyt': 63, 'best': 15}, {'pyt': 24, 'fun': 34}, {'pyt': 47, 'best': 64}, {'pyt': 13, 'fun': 55}]

Counting occurrences for key: pyt
The result is:
{13: 2, 63: 1, 24: 1, 47: 1}

Using itertools.groupby

You can also use groupby after sorting the data by the target key ?

from itertools import groupby

my_dict = [
    {'pyt': 13, 'fun': 44}, 
    {'pyt': 63, 'best': 15},
    {'pyt': 24, 'fun': 34}, 
    {'pyt': 47, 'best': 64},
    {'pyt': 13, 'fun': 55}
]

my_key = 'pyt'

# Sort by the key value first (required for groupby)
sorted_data = sorted(my_dict, key=lambda x: x[my_key])

# Group by key value and count occurrences
result = {key: len(list(group)) for key, group in groupby(sorted_data, key=lambda x: x[my_key])}

print("The result is:")
print(result)

The result is:
{13: 2, 24: 1, 47: 1, 63: 1}

Using Dictionary with Manual Counting

A simple manual approach using a dictionary to track counts ?

my_dict = [
    {'pyt': 13, 'fun': 44}, 
    {'pyt': 63, 'best': 15},
    {'pyt': 24, 'fun': 34}, 
    {'pyt': 47, 'best': 64},
    {'pyt': 13, 'fun': 55}
]

my_key = 'pyt'
result = {}

for dictionary in my_dict:
    if my_key in dictionary:
        value = dictionary[my_key]
        result[value] = result.get(value, 0) + 1

print("The result is:")
print(result)

The result is:
{13: 2, 63: 1, 24: 1, 47: 1}

Comparison

Conclusion

Use collections.Counter for the most efficient value counting. The manual dictionary approach offers flexibility, while groupby works best with pre-sorted data.