Difference between modes a, a+, w, w+, and r+ in built-in open function?

Question

In the python built-in open function, what is the exact difference between the modes w, a, w+, a+, and r+?

Answer 1

The options are the same as for the fopen function in the C standard library:

w truncates the file, overwriting whatever was already there

a appends to the file, adding onto whatever was already there

w+ opens for reading and writing, truncating the file but also allowing you to read back what's been written to the file

a+ opens for appending and reading, allowing you both to append to the file and also read its contents