Python program to convert Dict of list to CSV

A dictionary of lists is a common data structure where each key maps to a list of values. Converting this structure to CSV format is useful for data analysis and sharing tabular data.

Here's an example of a dictionary of lists ?

data = {
    'numbers': [1, 2, 3, 4, 5, 6],
    'states': ['UP', 'Tamil Nadu', 'Telangana', 'Gujarat', 'UP', 'Tamil Nadu'],
    'cities': ['Agra', 'Chennai', 'Hyderabad', 'Surat', 'Lucknow', 'Coimbatore']
}
print(data)

{'numbers': [1, 2, 3, 4, 5, 6], 'states': ['UP', 'Tamil Nadu', 'Telangana', 'Gujarat', 'UP', 'Tamil Nadu'], 'cities': ['Agra', 'Chennai', 'Hyderabad', 'Surat', 'Lucknow', 'Coimbatore']}

When converted to CSV format, this data becomes a tabular structure where keys become column headers and list values form rows ?

numbers,states,cities
1,UP,Agra
2,Tamil Nadu,Chennai
3,Telangana,Hyderabad
4,Gujarat,Surat
5,UP,Lucknow
6,Tamil Nadu,Coimbatore

Using the csv Module

The csv module provides built-in methods for writing CSV files. We use writerow() for headers and writerows() with zip() to transpose the data ?

import csv

def dict_to_csv_builtin(data, filename):
    """Convert dictionary of lists to CSV using csv module"""
    keys = list(data.keys())
    values = list(data.values())
    
    with open(filename, 'w', newline='') as csvfile:
        writer = csv.writer(csvfile)
        writer.writerow(keys)  # Write headers
        writer.writerows(zip(*values))  # Write data rows

# Sample data
data = {
    'numbers': [1, 2, 3, 4, 5],
    'colors': ['red', 'green', 'yellow', 'blue', 'brown'],
    'fruits': ['apple', 'avocado', 'pineapple', 'blueberry', 'date']
}

print("Input Dictionary:", data)
dict_to_csv_builtin(data, 'output_csv.csv')
print("CSV file created successfully!")

# Read and display the CSV content
with open('output_csv.csv', 'r') as f:
    content = f.read()
    print("\nCSV Content:")
    print(content)

Input Dictionary: {'numbers': [1, 2, 3, 4, 5], 'colors': ['red', 'green', 'yellow', 'blue', 'brown'], 'fruits': ['apple', 'avocado', 'pineapple', 'blueberry', 'date']}
CSV file created successfully!

CSV Content:
numbers,colors,fruits
1,red,apple
2,green,avocado
3,yellow,pineapple
4,blue,blueberry
5,brown,date

How zip(*values) Works

The zip(*values) function transposes the data from column-wise to row-wise format ?

# Understanding zip(*values)
values = [[1, 2, 3], ['red', 'green', 'blue'], ['apple', 'banana', 'cherry']]

print("Original values (column-wise):")
for i, col in enumerate(values):
    print(f"Column {i}: {col}")

print("\nAfter zip(*values) (row-wise):")
for i, row in enumerate(zip(*values)):
    print(f"Row {i}: {row}")

Original values (column-wise):
Column 0: [1, 2, 3]
Column 1: ['red', 'green', 'blue']
Column 2: ['apple', 'banana', 'cherry']

After zip(*values) (row-wise):
Row 0: (1, 'red', 'apple')
Row 1: (2, 'green', 'banana')
Row 2: (3, 'blue', 'cherry')

Using Pandas Library

Pandas provides a more convenient approach using DataFrame.to_csv(). It automatically handles the conversion and offers more formatting options ?

import pandas as pd

def dict_to_csv_pandas(data, filename):
    """Convert dictionary of lists to CSV using pandas"""
    df = pd.DataFrame(data)
    df.to_csv(filename, index=False)
    return df

# Sample data
data = {
    'nums': [1, 2, 3, 4, 5, 6],
    'states': ['UP', 'Tamil Nadu', 'Telangana', 'Gujarat', 'UP', 'Tamil Nadu'],
    'cities': ['Agra', 'Chennai', 'Hyderabad', 'Surat', 'Lucknow', 'Coimbatore']
}

print("Input Dictionary:", data)
df = dict_to_csv_pandas(data, 'output_pandas.csv')
print("\nDataFrame created:")
print(df)

# Display CSV content
with open('output_pandas.csv', 'r') as f:
    content = f.read()
    print("\nCSV Content:")
    print(content)

Input Dictionary: {'nums': [1, 2, 3, 4, 5, 6], 'states': ['UP', 'Tamil Nadu', 'Telangana', 'Gujarat', 'UP', 'Tamil Nadu'], 'cities': ['Agra', 'Chennai', 'Hyderabad', 'Surat', 'Lucknow', 'Coimbatore']}

DataFrame created:
   nums      states       cities
0     1          UP         Agra
1     2  Tamil Nadu      Chennai
2     3   Telangana    Hyderabad
3     4     Gujarat        Surat
4     5          UP      Lucknow
5     6  Tamil Nadu   Coimbatore

CSV Content:
nums,states,cities
1,UP,Agra
2,Tamil Nadu,Chennai
3,Telangana,Hyderabad
4,Gujarat,Surat
5,UP,Lucknow
6,Tamil Nadu,Coimbatore

Comparison

Handling Unequal List Lengths

When dictionary lists have different lengths, pandas handles it gracefully by filling missing values with NaN ?

import pandas as pd

# Dictionary with unequal list lengths
data_unequal = {
    'short': [1, 2],
    'medium': [10, 20, 30],
    'long': [100, 200, 300, 400]
}

df = pd.DataFrame(data_unequal)
print("DataFrame with unequal lengths:")
print(df)

DataFrame with unequal lengths:
   short  medium   long
0    1.0      10    100
1    2.0      20    200
2    NaN      30    300
3    NaN     NaN    400

Conclusion

Use the csv module for simple conversions with built-in Python capabilities. Choose pandas for complex data manipulation and when working within data analysis workflows. Both methods effectively convert dictionary of lists to CSV format.