QGIS » QGIS Application

Found a work-around: if you use a varchar to store the LabelX and LabelY then QGIS behaves as expected.

Hi Giovanni, yes it works well in the stable version (2.0.1). Labels of features with NULL values in X and Y fields are considered not fixed and are displayed in the default place.

Same behaviour when using memory data provider. Strangely, when creating memory layer, all goes well, but when loading it with memory layer saver when opening projects, Labels without XY disappear.
Confirmed on 2.0.1 and master.

With memory provider, there is no workaround with varchar.
Pretty serious issue since it ruins the intersest of plugin EasyCustomLabeling. it works once, but when reloading the project, all unpinned labels are masked

I guess everything is around NULL handling. On my memory layer working well, NULL are in fact objects <class 'PyQt4.QtCore.QPyNullVariant'>. When restored with memLayerSaver, I get <type 'NoneType'>.

Nathan pointed out the reasons of it: [[http://nathanw.net/2013/08/31/pyqgis-dealing-with-null-values/]]

So.. no idea why Postgis and SQLServer are concerned, but we could find a woraround patching mem layer saver to convert NULL to NULL objects. Transfering to Chris Crook.

Cheers,
Régis

Very odd! The MemoryLayerSaver (at least the most recent versions) uses a QDataStream to write the data. This correctly distinguishes between None and NULL. For example see the following code

from PyQt4.QtCore import *

file=QFile('/tmp/nulltest')
file.open(QIODevice.WriteOnly)
ds=QDataStream(file)
ds.setVersion(QDataStream.Qt_4_5)
ds.writeQVariant(None)
ds.writeQVariant(NULL)
ds.setDevice(None)
ds=None
file.close()

file.open(QIODevice.ReadOnly)
ds=QDataStream(file)
ds.setVersion(QDataStream.Qt_4_5)

v1=ds.readQVariant()
v2=ds.readQVariant()
print type(v1),v1
print type(v2),v2

This returns

<type 'NoneType'> None
<class 'PyQt4.QtCore.QPyNullVariant'> NULL

So it correctly identifies None and NULL as different.

On the other hand the python interface doesn't distinguish - None and NULL both get stored in the feature as NULL. As in this code

vl=QgsVectorLayer('POINT?field=f1:integer&field=f2:real&field=f3:string(20)','Test','memory')
dp=vl.dataProvider()
fields=dp.fields()
feat=QgsFeature(fields)
for i in ('f1','f2','f3'):
    feat[i]=NULL
    v1=feat[i]
    print type(v1),v1
    feat[i]=None
    v2=feat[i]
    print type(v2),v2

which returns

<class 'PyQt4.QtCore.QPyNullVariant'> NULL
<class 'PyQt4.QtCore.QPyNullVariant'> NULL
<class 'PyQt4.QtCore.QPyNullVariant'> NULL
<class 'PyQt4.QtCore.QPyNullVariant'> NULL
<class 'PyQt4.QtCore.QPyNullVariant'> NULL
<class 'PyQt4.QtCore.QPyNullVariant'> NULL

So the way it looks is that whether there is a None or NULL in the QgsFeature originally, when it is restored through the memory layer saver and installed into the QgsFeature it turns to NULL.

Which doesn't make sense with what you are saying, that NULLs are restored as None. On the other hand it does mean that Nones are restored as NULL.

I will fix this in the Memory layer saver - hopefully that will also sort out the labelling problem, even thought it looks like it is the wrong way around. It does look like there may be an issue with the QgsFeature SIP wrapping