Python Projects for Beginners a ten-Week Bootcamp Approach to Python Programming

Writing to CSV Files

CSV files work with data by separating a comma between each cell. This is known as a tabular data structure. To get started working with them, Python has a default library called “csv.” We’ll need to import that in order to work with them. After importing this library, we’ll use the second method of opening files using the “with” keyword. This concept works like a while loop, so that while the file is open, we can work with it, and once the block of code is done running, it closes the file automatically for us. Let’s check out the example:

1| # opening/creating and writing to a csv file
2| import csv 3| with open("test.csv", mode="w", newline="") as f:
4| writer = csv.writer(f, delimiter=",")
5| writer.writerow( ["Name", "City"] )
6| writer.writerow( ["Craig Lou", "Taiwan"] )


Go ahead and run the cell. Let’s walk through this line by line. We import the CSV library on line 2. Then we open the file in write mode as the variable f. We’ve also set the newline parameter to an empty string so that it doesn’t create empty lines between rows. On line 4, we create a writer variable that allows us to write to the CSV file. The last two lines write a couple lines of data to the CSV file. Once the block is complete, the file closes automatically, and we’re done. Go ahead and check out the file; you’ll see the new data output. Remember that write mode will always overwrite any data that was in the file previously.

Reading from CSV Files

In order to read the data from the CSV file we just created, we can simply set the mode to read:

1| # reading from csv files 2| with open("test.csv", mode="r") as f:
3| reader = csv.reader(f, delimiter=",") 4| for row in reader:
5| print(row)
ChapTer 6 DaTa ColleCTions anD Files
Go ahead and run the cell. You’ll notice that it outputs each row as a list with two items inside. We opened the file in read mode as the variable f. We then create a reader object through the CSV library which reads the contents in the file for us. Then we loop over the reader variable and print out each piece of data.
Note objects will be covered in a later week.