Python 3.14+osfunction

os.readinto()

os — file descriptors

Reads from a file descriptor into a pre-allocated mutable buffer (bytearray/memoryview) and returns the number of bytes read, without creating a new bytes object. Python 3.14+.

Syntax

os.readinto(fd, buffer, /)

It writes data straight into the given bytearray or memoryview and returns the byte count — no new bytes object per read. That saves memory and time in loops over large streams, especially with a memoryview slice over the free part of the buffer. Like os.read it may fill less than the buffer; 0 means end of file.

Examples

import os
r, w = os.pipe()
os.write(w, b'hello world')
buf = bytearray(5)
print(os.readinto(r, buf))   # → 5 (сколько байт реально прочитано)
print(bytes(buf))            # → b'hello'
print(os.readinto(r, bytearray(0)))   # → 0 (буфер нулевой длины)
os.close(r); os.close(w)

See also

Common errors

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