What is a Fixed-Width File?
💡 Definition: A fixed-width text file contains data that is structured in rows and columns. Each row contains one data entry consisting of multiple values (one value per column). Each column has a fixed width, i.e., the same number of characters, restricting the maximum data size per column.
Example of a Fixed-Width File
Here’s an example of the fixed-width file format as a screenshot of 'my_file.fwf'
with the abbreviated “Fixed Width File” suffix .fwf
:

Here’s the same file format for copy&pasting:
Name Age Job Income
Alice 24 Programmer 99000
Bob 28 Scientist 44000
Carl 44 Manager 129000
Dave 18 Secretary 29000
How to Convert a Fixed-Width File (.fwf) to a Comma-Separated Values (.csv) File?
The pure Python (no-library) way of converting a fixed-width file to a comma-separated values (CSV) file uses four main steps:
- Open the input (
.fwf
) and output (.csv
) files - Read the input data as a list of strings (one fixed-width string per line)
- Convert the list of fixed-width strings to a list of lists (one inner list per row)
- Write each row using a comma as a separator
Here’s the code solution to this problem:
# Open the input (.fwf) and output (.csv) files with open('my_file.fwf') as infile: with open('my_file.csv', 'w') as outfile: # Read the input data as list of strings data = infile.readlines() # Convert list of fixed-width strings to list of lists csv_data = [line.split() for line in data] # Write each row using comma as separator for row in csv_data: outfile.write(','.join(row) + '\n')
If the input file looks like this:

The resulting CSV file looks like this:

How to Convert a Fixed-Width File to a CSV in Pandas?
You can convert a fixed-width file to a CSV using Python pandas by reading the fixed-width file as a DataFrame df
using pd.read('my_file.fwf')
and writing the DataFrame to a CSV using df.to_csv('my_file.csv', index=False)
.
Here’s the code solution:
import pandas as pd df = pd.read_fwf('my_file.fwf') df.to_csv('my_file.csv', index=False)
Again, if the input file looks like this:

The resulting CSV file looks like this:

🌍 Related Tutorial: Understanding the DataFrame df.to_csv()
method

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