[PyQt] QSettings with different Python versions

Phil Thompson phil at riverbankcomputing.com
Tue Apr 16 12:45:02 BST 2013

On Wed, 10 Apr 2013 18:49:24 +0200, Tobias Rzepka
<tobias.rzepka at gmail.com>
> Phil Thompson schrieb am 09.04.2013 15:45:
>> On Fri, 05 Apr 2013 16:20:58 +0200, Tobias Rzepka
>> <tobias.rzepka at gmail.com>
>> wrote:
>>> Hello Phil,
>>> I have encountered some odd behavior in the QSettings if I use the
>>> ini with Python 2 and Python 3. I was able to track the problem down
>>> a entry which is a complex list with dictionaries in it (in the
>>> the ViewProfiles entry from Eric5).
>>> There are two solutions I've found:
>>> 1. Delete the ViewProfiles entry
>>> 2. Access the ini without a default parameter of the QSettings.value
>> method
>>> Detlev and I would like to implement the second solution because users
>>> who use a old (<=5.3) and new version of Eric5 with different
>>> interpreter should see merely no differences in there settings.
>>> The funny thing is, thats not only the default parameter of the value
>>> method but also the surrounding. I've to store the settings in a class
>>> (Prefs) and the second parameter has to be from another class, like in
>>> the example.
>>> And if you uncomment the line SingleApplicationMode=false, everything
>>> fine again (but also no solution, because the next access is going to
>>> fail).
>>> Can you tell, if it's a odd behavior of QSettings or maybe of Python?
>>> I've attached a ini and a little code snippet from eric5, to
>>> the behavior. The error message is the following:
>>> Traceback (most recent call last):
>>>      File "D:\Projekte\Eric5\qsettings\py2_qsettings.py", line 54, in
>>>      <module>
>>>        Prefs2.uiDefaults['SingleApplicationMode'])
>>>      File "C:\Python27\lib\encodings\__init__.py", line 100, in
>>>      search_function
>>>        level=0)
>>>      File "C:\Python27\lib\pickle.py", line 1382, in loads
>>>        return Unpickler(file).load()
>>>      File "C:\Python27\lib\pickle.py", line 858, in load
>>>        dispatch[key](self)
>>>      File "C:\Python27\lib\pickle.py", line 886, in load_proto
>>>        raise ValueError, "unsupported pickle protocol: %d" % proto
>>> ValueError: unsupported pickle protocol: 3
>>> Thanks. Tobias
>>> By the way: If I manage to read the ini and save it again, Python 2
>>> removes the quotation marks around the ViewProfiles values.
>>> the settings file isn't readable in Python 3...
>> Have you made your example too simple? It works fine for me.
>> Phil
> Hello Phil,
> I've tried it now on three different machines and different Python2 / 
> PyQt4 versions and all of them showed the exception.
> I've tried
> 1.
> Win7, 64bit, Py2.7.3 64bit, PyQt4 4.9.4
> Win7, 64bit, Py2.7.4 32bit, PyQt4 4.10
> 2.
> Win7, 64bit, Py2.7.3 32bit, PyQt4 4.9.5
> 3.
> Win7, 32bit, Py2.7.3 32bit, PyQt4 4.9.4
> No machine have a development environment for PyQt. All of them use the 
> GPL version of your installer.
> Maybe that's the difference?
> *Maybe it's the way you called my script:* if I run it from command line

> (only tested with Win7, 64bit, Py2.7.4 32bit, PyQt4 4.10) like
> c:\python27\python.exe py2_qsettings.py
> it throws the exception, if I use execfile('py2_qsettings.py') it works 
> flawless!

PyQt uses the default pickle protocol to handle serialising objects that
it has no explicit support for. The default protocol for Python3 is
incompatible with the default for Python2.

I could add a pyqtSetPickleProtocol() function to QtCore to allow an
application to set a compatible protocol.


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