CSV Files with the csv Module

Hello students, welcome to Kwickprep. Your school keeps marks in a spreadsheet. Can a Python program read that table and add new rows to it? Yes, with CSV files. Today we will write and read CSV files using the csv module, and avoid the most common mistakes.

First, the new term. CSV stands for comma separated values. A CSV file is just a plain text file, so any text editor can open it. Each line in the file is one row of a table. Inside a line, commas separate the values into columns. Unlike a binary file, such as a photo, you can open it in Notepad and read it.

Open a CSV file in a spreadsheet, like Excel or Google Sheets, and it looks like a neat table. The first line, Name comma Marks, becomes the heading row. The line Riya comma eighty eight becomes Riya in column A and eighty eight in column B. Aman's line becomes the next row in the same way. That is why offices, banks and schools share data as CSV files.

We could split lines with commas ourselves, but it is risky. A module is a file of ready-made Python code that comes with Python. The line import csv loads the csv module. It adds quotes when a value itself has a comma, like an address. When reading, it splits every line into a list for us.

Let us write a CSV file. We open m dot csv with the mode w, and newline equals empty quotes, which we explain soon. The csv dot writer function makes a writer object, which we call w. The writerow method takes one list and writes it as one row. It puts the comma in for us. After closing, we open the file again with the with statement and print it.

When you have many rows, use writerows, with an s at the end. It takes a list of lists. Each inner list becomes one row. Here, the list called rows has two inner lists, so the file gets two lines. This time we made the writer and used it in the same line.

Here is why the module is safer. The city value, Pune comma M H, has a comma inside it. If we wrote it by hand, a spreadsheet would split it into two columns. The writer puts double quotes around that value, so it stays in one column.

Now let us read. First we create a small file with a heading line and one student. We open it with newline equals empty quotes again. The csv dot reader function gives the rows one by one, so we loop over it with for. Each row comes out as a list of values. Look carefully, eighty eight has quotes around it.

The reader always gives strings, even for numbers. The next function takes just one row from the reader. Pause and predict. What is row one plus row one? It is not one hundred seventy six. Adding two strings joins them, so it prints eight eight eight eight. Convert with int first, and you get one hundred seventy six.

Here is a case based program. The teacher gives five grace marks to every student. The list function turns all rows from the reader into one list, called rows. Row zero is only headings, so rows from index one skips it. For each student, we print the name, and the marks as an int plus five. Om gets ninety three, and Anu gets eighty one.

Now, why newline equals empty quotes? The csv writer ends every row with its own line ending characters. Without this argument, Windows adds one more line ending on top. Open that file in a spreadsheet, and you see a blank row after every row. So always open CSV files with newline equals empty quotes, for writing and for reading.

Let us list the mistakes students make most. Forget newline equals empty quotes, and you may get blank rows. Open in mode w to add a student, and all old rows are erased, so use mode a to append. Add a number to a string value, and you get a type error, so convert with int first. Forget import csv, and you get a name error. Write csv dot write instead of writer, and you get an attribute error.

Let us revise what we learned today. A CSV file is plain text where commas split the columns. Writerow writes one row, and writerows writes many. The reader gives each row as a list of strings. Always open with newline equals empty quotes to avoid blank rows. Mode a adds rows, but mode w erases them. Try making a CSV file of your own class timetable.