cheat sheets.

Mode |  Meaning
-----+--------------------------------------------------------
"r"  |  Read-only, starts at beginning of file  (default mode).
-----+--------------------------------------------------------
"r+" |  Read-write, starts at beginning of file.
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"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).