"r"   Opens a file for reading only.
"r+"  Opens a file for both reading and writing.
"rb"  Opens a file for reading only in binary format.
"rb+" Opens a file for both reading and writing in binary format.
"w"   Opens a file for writing only.
"a"   Open for writing.  The file is created if it does not exist.
"a+"  Open for reading and writing.  The file is created if it does not exist.