File access mode description

file-mode.txt

Mode |  Meaning
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"r"  |  Read-only, starts at beginning of file  (default mode).
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"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.
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"w+" |  Read-write, truncates existing file to zero length
     |  or creates a new file for reading and writing.
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"a"  |  Write-only, starts at end of file if file exists,
     |  otherwise creates a new file for writing.
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"a+" |  Read-write, starts at end of file if file exists,
     |  otherwise creates a new file for reading and
     |  writing.
-----+--------------------------------------------------------
 "b" |  Binary file mode (may appear with
     |  any of the key letters listed above).
     |  Suppresses EOL <-> CRLF conversion on Windows. And
     |  sets external encoding to ASCII-8BIT unless explicitly
     |  specified.
-----+--------------------------------------------------------
 "t" |  Text file mode (may appear with
     |  any of the key letters listed above except "b").