New submission from Alexey Izbyshev <izbys...@ispras.ru>:
The fact that "buffering=1" is usable only in text mode is documented for
open(). In binary mode, setting buffering to 1 is silently ignored and
equivalent to using default buffer size. I argue that such behavior is:
1. Inconsistent
For example, attempts to use buffering=0 in text mode or to specify encoding in
binary mode raise ValueError.
2. Dangerous
Consider the following code fragment running on *nix (inspired by real-world
code):
with open("fifo", "wb", buffering=1) as out:
for line in lines:
out.write(line)
"fifo" refers to a FIFO (named pipe). For each line, len(line) <= PIPE_BUF.
Because of line buffering, such code must be able to safely assume that each
write() is atomic, so that even in case of multiple writers no line read by a
FIFO reader will ever get intermixed with another line. And it's so in Python
2. After migration to Python 3 (assuming that line is now bytes), this code
still works on Linux because of two factors:
a) PIPE_BUF is 4096, and so is the default IO buffer size (usually)
b) When a write() is going to overflow the buffer, BufferedWriter first flushes
and then processes the new write() (based on my experiment). So, each OS
write() is called with complete lines only and is atomic per (a).
But PIPE_BUF is 512 on Mac OS X (I don't know about default buffer size). So,
we are likely to have a subtle 2-to-3 migration issue.
I suggest to raise a ValueError if buffering=1 is used for binary mode. Note
that issue 10344 (msg244354) and issue 21332 would have been uncovered earlier
if it was done from the beginning.
----------
components: IO
messages: 307775
nosy: izbyshev, pitrou
priority: normal
severity: normal
status: open
title: open() shouldn't silently ignore buffering=1 in binary mode
type: behavior
versions: Python 3.7
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue32236>
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