Persistence mode {‘r’, ‘r+’, ‘a’, ‘w’, ‘w-‘}

[SabineEmbacher]

mode:{‘r’, ‘r+’, ‘a’, ‘w’, ‘w-‘}, optional
Persistence mode: ‘r’ means read only (must exist); ‘r+’ means read/write (must exist); ‘a’ means read/write (create if doesn’t exist); ‘w’ means create (overwrite if exists); ‘w-‘ means create (fail if exists).