Why truncate when we open a file in 'w' mode in python

Question

I am going through Zed Shaw\'s Python Book. I am currently working on the opening and reading files chapters. I am wondering why we need to do a truncate, when we are alread

Answer 1

While it's not useful to truncate when opening in 'w' mode, it is useful in 'r+'. Though that's not the OP's question, I'm going to leave this here for anyone who gets lead here by Google as I did.

Let's say you open (with mode 'r+', remember there is no 'rw' mode) a 5 line indented json file and modify the json.load-ed object to be only 3 lines. If you target.seek(0) before writing the data back to the file, you will end up with 2 lines of trailing garbage. If you target.truncate() it you will not.

I know this seems obvious, but I'm here because I am fixing a bug that occurred after an object that stayed the exact same size for years... shrank because of a signing algorithm change. (What is not obvious is the unit tests I had to add to prevent this in the future. I wrote my longest docstring ever explaining why I'm testing signing with 2 ridiculously contrived algorithms.)

Hope this helps someone.