Python - Convert list of dictionaries to Dictionary Value list

We are given a list of dictionaries we need to convert it to dictionary. For example, given a list of dictionaries: d = [{'a': 1, 'b': 2}, {'a': 3, 'b': 4}, {'a': 5, 'b': 6}], the output should be: {'a': [1, 3, 5], 'b': [2, 4, 6]}.

Using Dictionary Comprehension

Using dictionary comprehension, we can convert a list of dictionaries into a dictionary of value lists by iterating over the keys of the first dictionary to define the keys of the result.

d = [
    {'name': 'Alice', 'age': 25, 'city': 'New York'},
    {'name': 'Bob', 'age': 30, 'city': 'Los Angeles'},
    {'name': 'Charlie', 'age': 35, 'city': 'Chicago'}
]

res = {key: [d[key] for d in d] for key in d[0].keys()}
print(res)

{'name': ['Alice', 'Bob', 'Charlie'], 'age': [25, 30, 35], 'city': ['New York', 'Los Angeles', 'Chicago']}

Explanation:

• Outer comprehension iterates over the keys of the first dictionary to define the result's keys.
• Inner comprehension collects values for each key from all dictionaries in the list.

Using defaultdict from collections

Using defaultdict from collections, we can convert a list of dictionaries into a dictionary of value lists by appending values to lists for each key dynamically. This approach handles keys efficiently without requiring prior initialization.

from collections import defaultdict

d = [
    {'name': 'Alice', 'age': 25, 'city': 'New York'},
    {'name': 'Bob', 'age': 30, 'city': 'Los Angeles'},
    {'name': 'Charlie', 'age': 35, 'city': 'Chicago'}
]

res = defaultdict(list)
for d in d:
    for key, value in d.items():
        res[key].append(value)

print(dict(res))

{'name': ['Alice', 'Bob', 'Charlie'], 'age': [25, 30, 35], 'city': ['New York', 'Los Angeles', 'Chicago']}

Explanation:

• Res dictionary is initialized as a defaultdict with list, allowing each key to automatically have an empty list as its default value.
• For each dictionary in the list, the inner loop iterates through its key-value pairs, appending each value to the corresponding key's list in res

Using pandas

Using pandas, we can convert a list of dictionaries into a DataFrame and then use the values.tolist() method to extract the values as a list of lists. This approach efficiently handles structured data and converts it into the desired format.

import pandas as pd

# Define a list of dictionaries
d = [
    {'name': 'Alice', 'age': 25, 'city': 'New York'},
    {'name': 'Bob', 'age': 30, 'city': 'Los Angeles'},
    {'name': 'Charlie', 'age': 35, 'city': 'Chicago'}
]

# Convert the list of dictionaries into a pandas DataFrame
df = pd.DataFrame(d)

# Convert the DataFrame into a dictionary with lists of values for each column using 'orient="list"'
res = df.to_dict(orient='list')

# Print the resulting dictionary
print(res)

{'name': ['Alice', 'Bob', 'Charlie'], 'age': [25, 30, 35], 'city': ['New York', 'Los Angeles', 'Chicago']}

Explanation:

• List of dictionaries is converted into a pandas DataFrame, where each dictionary becomes a row and each key becomes a column header.
• DataFrame is then converted to a dictionary using to_dict(orient='list'), where each column name maps to a list of its corresponding values.