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

[Daniel P . Berrangé]

On Thu, Sep 16, 2021 at 09:42:30AM -0400, John Snow wrote:
> 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?

Well this machine.py is using UNIX domain sockets and FD passing,
so Windows is out of the question.

I'd be inclined to just keep it simple unless someone reports a
real bug with it.


Regards,
Daniel
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