When to open file in binary mode (b)?

Question

I noticed in the docs they always open a CSV file with ‘wb’. Why the ‘b’? I know b stands for binary mode, but when do you use binary mode (I’d guess CSV file is not binary). If relevant I’m writing to the CSV from results from query by arcpy.da.SearchCursor()

EDIT: just noticed according to this answer wb+ is used for writing a binary file. What does including the + do?

Answer 1

Use 'b' mode, to read/write binary data as is without any transformations such as converting newlines to/from platform-specific values or decoding/encoding text using a character encoding.

csv module is special. csv data is text and therefore the text mode would be expected but csv module uses '\r\n' by default to terminate rows on all platforms and it always recognizes both '\r' and '\n' as newlines. If you open the corresponding file in the text mode (with universal newlines) then you will get '\r\r\n' (corrupted newlines) on Windows (os.linesep == '\r\n' there). That is why Python 2 docs say that you must use the binary mode. In Python 3, the text mode is used but you should pass newline='' to disable universal newlines mode. You would also want to disable universal newlines if you want to preserve possible newline characters (such as '\r') embedded in fields.

Answer 2

File open default is to use text mode, which may convert '\n' characters to a platform-specific representation on writing and back on reading.

In windows this will modify the line breaks from '\n' to '\r\n' which will create problem opening the CSV file in other applications/platforms.

Thus, when opening a binary file, you should append 'b' to the mode value to open the file in binary mode, which will improve portability. On systems that don’t have this distinction, adding the 'b' has no effect.

Note: 'w+' truncates the file.

Modes 'r+', 'w+' and 'a+' open the file for updating (reading and writing).

As detailed here: https://docs.python.org/2/library/functions.html#open