[New-bugs-announce] [issue34070] Superfluous call to isatty in open() when buffering >= 0

[David Herberth]

New submission from David Herberth <python at dav1d.de>:

_iomodule.c:_io_open_impl checks for isatty even if not necessary (when buffering >= 0).

Code: https://github.com/python/cpython/blob/c0ee341b29bd7d978b49272a2c0e2dcfa77404d5/Modules/_io/_iomodule.c#L392

    {
        PyObject *res = _PyObject_CallMethodId(raw, &PyId_isatty, NULL);
        if (res == NULL)
            goto error;
        isatty = PyLong_AsLong(res);
        Py_DECREF(res);
        if (isatty == -1 && PyErr_Occurred())
            goto error;
    }

    if (buffering == 1 || (buffering < 0 && isatty)) {
        buffering = -1;
        line_buffering = 1;
    }
    else
        line_buffering = 0;


Python Code to reproduce:

with open('foo', 'rb', buffering=0) as f:
    f.read()


This generates an error (can be seen with strace):

ioctl(5, TCGETS, 0x7ffef1435b60)  = -1 ENOTTY (Inappropriate ioctl for device)

I'll open a PR shortly.

----------
components: IO
messages: 321281
nosy: Dav1d
priority: normal
severity: normal
status: open
title: Superfluous call to isatty in open() when buffering >= 0
type: performance
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34070>
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