How to read/write CSV files in Python

In this tutorial, we’re gonna look at way to use csv module to read, write CSV files in Python program.

CSV files

CSV stands for ‘comma-separated values’. CSV file is a spreadsheet stored as plain-text file.
Each line in a CSV file represents a row, and commas separate cells in the row.

For example:

Instead of commas, cells could be separated with tab characters:

CSV file is simple than Excel spreadsheet because it doesn’t have:
– types, everything is just a string
– font size or color
– multiple worksheets
– cell widths and heights
– merge cells
– images or charts

Read CSV file in Python

You can follow these steps to read data from a CSV file:
– import csv module
– open CSV file using open() function
– create a Reader object that lets you iterate over rows
– access values in the Reader object by:
+ convert it to a plain Python list using list() function
+ use for-loop (avoid loading the entire file into memory at once)

Remember that we can loop over Reader object only once.
To reread the CSV file by for-loop, we must create Reader object again.

Write CSV file in Python

You can follow these steps to write data to a CSV file:
– import csv module
– create new CSV file using open() function in write mode
– create a Writer object using csv.writer() function
– use Writer object writerow() method to write a line (form of a list). The return value of writerow() is the number of characters for that row (including newline characters).

Result in grokonez.csv file:

*Note: On Windows, newline keyword argument of open() function helps us avoid double-spaced when writing a new row in the output CSV file.

If we want to separate cells and end rows with a specific character, just pass delimiter and lineterminator as keyword arguments of csv.writer() function: