"r" - Read - Default value. Opens a
file for reading, error if the file does not exist

"a" - Append - Opens a file for
appending, creates the file if it does not exist

"w" - Write - Opens a file for writing,
creates the file if it does not exist

"x" - Create - Creates the specified file, returns
an error if the file exists

"t" - Text - Default value. Text mode

"b" - Binary - Binary mode (e.g.
images)