cheat sheets.

"r"     Read-only, starts at beginning of file (default mode).
"r+"    Read-write, starts at beginning of file.
"w"     Write-only, truncates existing file to zero length or creates a new file
for writing.
"w+"    Read-write, truncates existing file to zero length or creates a new file
for reading and writing.
"a"     Write-only, starts at end of file if file exists, otherwise creates a
new file for writing.
"a+"    Read-write, starts at end of file if file exists, otherwise creates a
new file for reading and writing.
"b"     (DOS/Windows only) Binary file mode (may appear with any of the key
letters listed above).