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Buffering

buffering

Buffered I/O collects the data in memory before writing it out. flush() forces the buffer out.

Syntax

open(file, buffering=-1)  # -1 авто, 0 без буфера (только binary), 1 строчный
stream.flush()

buffering=0 is allowed in binary mode only — in text mode open() raises ValueError, while line buffering (buffering=1) makes sense only for text writing. The buffer sits in process memory, so a crash or os._exit() loses whatever was not flushed, and flush() itself only passes the data to the OS; getting it onto the actual disk takes os.fsync(). For console output the same effect is available as print(..., flush=True).

Examples

import io
buf = io.StringIO()
buf.write('text')
buf.getvalue() # → 'text'  StringIO не нуждается в flush
f = open('test_io.txt', 'w', buffering=1)
f.write('line\n')
f.flush()  # строки сразу сбрасываются
f.close()
open('test_io.txt').read() # → 'line\n'

See also

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