Many tutorials on writing csv files have the mode set to 'wb', why?

Question

For example: Writing to csv file in Python

with open('StockPrice.csv', 'wb') as f:

Why would we need to open in binary for a csv file?

Is this just habit, or is there a use-case for when binary is necessary for a csv file?

Answer 1

It's necessary to use the mode "wb" when writing output using the csv module on Windows, because the csv module will write out line-ends as \r\n regardless of what platform you're running on.

If you're running on Windows, and you have a file open with mode "w", Python will add an extra carriage return every time you write a newline. So if you use a file with mode "w" to write output using the csv module, you will end up with \r\r\n line-endings, as both Python and the csv module have added carriage-return characters.

Here's a quick program that demonstrates the result. Note that we read the file in binary mode ("rb") to prevent Python from replacing \r\n with \n as it reads the file back in:

import csv

with open("output.csv", "w") as f:
    w = csv.writer(f)
    w.writerow([1,2,3,4])
    w.writerow([5,6,7,8])

with open("output.csv", "rb") as f:
    print repr(f.read())

When I run this on Windows, I get the following output:

'1,2,3,4\r\r\n5,6,7,8\r\r\n'