Message 198230 - Python tracker

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Author	terry.reedy
Recipients	Arfrever, chortos, gvanrossum, petri.lehtinen, pitrou, python-dev, r.david.murray, serhiy.storchaka, terry.reedy, vstinner
Date	2013-09-21.23:02:07
SpamBayes Score	-1.0
Marked as misclassified	Yes
Message-id	<1379804527.99.0.805331581279.issue12085@psf.upfronthosting.co.za>
In-reply-to

Content
The message problem can arise during exit if __del__ depends an any attribute of any object. It is hard to imagine a __del__ method that does not. Any __del__ method, including that of Popen, could handle AttributeErrors by wrapping the whole body in try: <body> except AttributeError: pass The is essentially what is done by the code that calls __del__, except that 'pass' is replaced by "print(message, file=sys.stderr)". If we patch Popen at all, I think this try:except should be the fix, not a class attribute. To explain what I meant by the class attribute hack being tricky, consider the original version of Popen.__del__, minus the comments. def __del__(self, _maxsize=sys.maxsize, _active=_active): if not self._child_created: return self._internal_poll(_deadstate=_maxsize) if self.returncode is None and _active is not None: _active.append(self) Since self._internal_poll is an instance method, it is not a problem. But what about the self.returncode data attribute? Should we also add a class 'returncode' attribute? If so, what should be its value? None? or object()? Or is it guaranteed that when _child_created is set true, returncode will be defined, so that a class attribute is not needed? I don't know the answers to these questions. Popen.__init__ is about 130 lines and self._child_created is set to True in two other methods. I did not look where returncode is set, but it is not near the spots where _child_created is set True.

The message problem can arise during exit if __del__ depends an any attribute of any object. It is hard to imagine a __del__ method that does not. Any __del__ method, including that of Popen, could handle AttributeErrors by wrapping the whole body in

try:
  <body>
except AttributeError:
  pass

The is essentially what is done by the code that calls __del__, except that 'pass' is replaced by "print(message, file=sys.stderr)".

If we patch Popen at all, I think this try:except should be the fix, not a class attribute.

To explain what I meant by the class attribute hack being tricky, consider the original version of Popen.__del__, minus the comments. 

    def __del__(self, _maxsize=sys.maxsize, _active=_active):
        if not self._child_created:
            return
        self._internal_poll(_deadstate=_maxsize)
        if self.returncode is None and _active is not None:
            _active.append(self)

Since self._internal_poll is an instance method, it is not a problem. But what about the self.returncode data attribute? Should we also add a class 'returncode' attribute? If so, what should be its value? None? or object()? Or is it guaranteed that when _child_created is set true, returncode will be defined, so that a class attribute is not needed?

I don't know the answers to these questions. Popen.__init__ is about 130 lines and self._child_created is set to True in two other methods. I did not look where returncode is set, but it is not near the spots where _child_created is set True.

History
Date	User	Action	Args
2013-09-21 23:02:08	terry.reedy	set	recipients: + terry.reedy, gvanrossum, pitrou, vstinner, Arfrever, r.david.murray, chortos, python-dev, petri.lehtinen, serhiy.storchaka
2013-09-21 23:02:07	terry.reedy	set	messageid: <1379804527.99.0.805331581279.issue12085@psf.upfronthosting.co.za>
2013-09-21 23:02:07	terry.reedy	link	issue12085 messages
2013-09-21 23:02:07	terry.reedy	create