[PyQt] QVariant and Python list

[Laurent Léonard]

Le vendredi 30 mai 2008 à 13:38, Phil Thompson a écrit :
> On Wednesday 28 May 2008 6:35:32 pm Laurent Léonard wrote:
> > Hi,
> >
> > I read QList and QMap are not implemented in PyQt, because of the
> > presence of Python lists and dictionaries.
> >
> > But how can I use QVariant to "encapsulate" Python lists and dictionaries
> > ? When I try to do it I get the following error message : "TypeError:
> > argument 1 of QVariant() has an invalid type"
>
> I'd be surprised if you got that error with the current version.
>
> There are a number of issues here which I'd like feedback on...
>
> The various QVariant ctors allow type convertors to have an effect, ie. in
> SIP terms they don't have the /Constrained/ annotation. For example,
> QVariant(const QStringList &) will accept a Python list of strings. I think
> this is a mistake as QVariant([]) will be treated as an empty QStringList
> but QVariant([0]) will be treated as a Python object.
>
> So I think every QVariant ctor that takes a Qt class should have
> /Constrained/ applied.
>
> However that would then mean that QVariant([]) would now be treated as an
> empty QList<QVariant> and still not as a Python object.
>
> For similar reasons QVariant({"abc": 0}) would be treated as a QMap, but
> QVariant({1: 0}) would be treated as a Python object.
>
> Therefore I also want to drop support for QVariant(QList<>) and
> QVariant(QMap<>) completely.
>
> The advantage is that anything that is not a fundamental type (int, float
> etc) or a wrapped type will be treated as a Python object and can be
> retrieved using toPyObject() - all very consistent.
>
> The disadvantages are that it is an incompatible change. It also becomes
> impossible to create a QVariant(QList<>) or QVariant(QMap<>) from Python
> (although I can't find a use case in Qt). I could add fromQVariantList()
> and fromQVariantMap() methods to get round the second problem.
>
> Comments?
>
> Phil
>
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Some interesting tests about QVariants and lists... 

>>> list = ["one", "two", "three"]
>>> list2 = [1, 2, 3]
>>> print QtCore.QVariant(list).toList()[0].toString()
one
>>> print QtCore.QVariant(list2).toList()[0].toString()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: argument 1 of QVariant() has an invalid type
>>> list2 = [QtCore.QVariant(1), QtCore.QVariant(2), QtCore.QVariant(3)]
>>> print QtCore.QVariant(list2).toList()[0].toString()
1

And about QVariants and dictionaries.... 

>>> dict = {"blahKey": QtCore.QVariant("blahBlahValue")}
>>> print QtCore.QVariant(dict).toMap()["blahKey"].toString()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'blahKey'
>>> print QtCore.QVariant(dict).toMap()[QtCore.QString("blahKey")].toString()
blahBlahValue

So for lists encapsulated into QVariant, the list have to contain strings, 
QStrings or QVariants. And for dictionaries encapsulated into QVariant, the 
keys have to be strings or QStrings and the values strings, QStrings or 
QVariants. But in both cases, QVariant use QStrings to store strings. 

-- 
Laurent Léonard
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