"r"  =  open for reading
"w"  =  erase or create new empty
"a"  =  append or create for writing
"rb" =  open binary file for reading
"wb" =  erase binary file or create empty
"ab" =  open binary file for appending
"r+" =  open for reading and writing
"w+" =  erase, open for reading/writing
"a+" =  retain contents, read/write
"r+b" or "rb+"  =  open binary file for read/write
"w+b" or "wb+"  =  erase binary file, read/write
"a+b" or "ab+"  =  retain contents of binary file, read/write