Re: [PATCH v3 1/1] python: Update for pylint 2.10

[John Snow <jsnow@redhat.com>]

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On Thu, Sep 16, 2021 at 8:59 AM Daniel P. Berrangé <berrange@redhat.com>
wrote:

> On Wed, Sep 15, 2021 at 11:40:31AM -0400, John Snow wrote:
> > A few new annoyances. Of note is the new warning for an unspecified
> > encoding when opening a text file, which actually does indicate a
> > potentially real problem; see
> > https://www.python.org/dev/peps/pep-0597/#motivation
> >
> > Use LC_CTYPE to determine an encoding to use for interpreting QEMU's
> > terminal output. Note that Python states: "language code and encoding
> > may be None if their values cannot be determined" -- use a platform
> > default as a backup.
> >
> > Signed-off-by: John Snow <jsnow@redhat.com>
> > ---
> >  python/qemu/machine/machine.py | 9 ++++++++-
> >  python/setup.cfg               | 1 +
> >  2 files changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/python/qemu/machine/machine.py
> b/python/qemu/machine/machine.py
> > index a7081b1845..51b6e79a13 100644
> > --- a/python/qemu/machine/machine.py
> > +++ b/python/qemu/machine/machine.py
> > @@ -19,6 +19,7 @@
> >
> >  import errno
> >  from itertools import chain
> > +import locale
> >  import logging
> >  import os
> >  import shutil
> > @@ -290,8 +291,14 @@ def get_pid(self) -> Optional[int]:
> >          return self._subp.pid
> >
> >      def _load_io_log(self) -> None:
> > +        # Assume that the output encoding of QEMU's terminal output
> > +        # is defined by our locale. If indeterminate, use a platform
> default.
> > +        _, encoding = locale.getlocale()
> > +        if encoding is None:
> > +            encoding = locale.getpreferredencoding(do_setlocale=False)
>
> Do we really need this getpreferredencoding ?  IIUC, this is a sign
> that the application is buggy by not calling
>
>   locale.setlocale(locale.LC_ALL, '')
>
> during its main() method, which I think we can just delegate to the
> code in question to fix. Missing setlocale will affect everything
> they run, so doing workarounds in only 1 place is not worth it IMHO
>
>
I genuinely don't know! (And, I try to keep the Python code free from
assuming Linux as much as I can help it.)

Python's getlocale documentation states: "language code and encoding may be
None if their values cannot be determined."
https://docs.python.org/3/library/locale.html#locale.getlocale

But it is quiet as to the circumstances under which this may happen.
Browsing the cpython source code, (3.9ish):

```
def getlocale(category=LC_CTYPE):
    localename = _setlocale(category)
    if category == LC_ALL and ';' in localename:
        raise TypeError('category LC_ALL is not supported')
    return _parse_localename(localename)
```
_setlocale is ultimately a call to (I think) _localemodule.c's
PyLocale_setlocale(PyObject *self, PyObject *args) C function.
It calls `result = setlocale(category, locale)` where the category is going
to be LC_CTYPE, so this should be equivalent to locale(3) (LC_CTYPE, NULL).

locale(3) says that "The return value is NULL if the request cannot be
honored."

Python parses that string according to _parse_localename, which in turn
calls normalize(localename).
Normalization looks quite involved, but has a fallback of returning the
string verbatim. If the normalized locale string is "C", we return the
tuple (None, None)!

So I figured there was a non-zero chance that we'd see a value of `None`
here.

Source code is in cpython/Lib/locale.py and cpython/Modules/_localemodule.c
if you want to nose around yourself.

I also have no idea how this will all shake out on Windows, so I decided to
add the fallback here just in case. (Does the Python package work on
Windows? I don't know, but I avoid assuming it won't EVER run there...
Certainly, I have an interest in having the QMP packages I am building work
on all platforms.)

Thoughts?

>          if self._qemu_log_path is not None:
> > -            with open(self._qemu_log_path, "r") as iolog:
> > +            with open(self._qemu_log_path, "r",
> > +                      encoding=encoding) as iolog:
> >                  self._iolog = iolog.read()
> Regards,
> Daniel
> --
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>
>

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