[PyQt] QVariant and Python list

Phil Thompson phil at riverbankcomputing.com
Fri May 30 12:38:48 BST 2008

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/ 

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.



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