Custom Python function to access symbol color crashes QGIS

[thymaro]

What is the bug or the crash?

User Feedback

I entered a function in the function editor and loaded it:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def symbol_color(layername, feature, parent):
    layers = QgsProject.instance().mapLayersByName(layername)
    if not layers:
        return "Layer not found"
    
    layer = layers[0]
    renderer = layer.renderer()
    context = QgsRenderContext()
    symbol = renderer.symbolForFeature(feature, context)
    if symbol:
        values = symbol.color().getRgb()
        return "{}, {}, {}, {}".format(*values)
    else:
        return "Symbol not found"

then I went back to the expression builder and typed

symbol_color( @layer_name )

QGIS crashed before the last letter in '@layer_name' was even displayed.

Report Details

Python Stack Trace

Windows fatal exception: access violation

Current thread 0x00002670 (most recent call first):
  File "C:\OSGeo4W/apps/qgis-ltr/./python\qgis\core\additions\qgsfunction.py", line 74 in func
    return self.function(*values, **kwvalues)

Stack Trace

QgsExpression::evaluate :
QgsCategorizedSymbolRenderer::valueForFeature :
QgsCategorizedSymbolRenderer::originalSymbolForFeature :
pdal::PointView::table :
PyLong_FromString :
PyObject_Vectorcall :
PyObject_Vectorcall :
PyEval_EvalFrameDefault :
PyFunction_Vectorcall :
PyArg_CheckPositional :
PyObject_Call :
PyObject_Call :
PyInit_sip :
PyInit_sip :
pdal::StreamPointTable::reset :
pdal::MetadataNode::value :
QgsExpressionFunction::run :
pdal::MetadataNode::value :
QgsExpressionNodeFunction::evalNode :
QgsExpressionNode::eval :
QgsExpression::evaluate :
QgsExpressionPreviewWidget::refreshPreview :
QgsExpressionBuilderWidget::txtExpressionString_textChanged :
QObject::qt_static_metacall :
QsciScintilla::handleModified :
QsciScintilla::qt_static_metacall :
QObject::qt_static_metacall :
QsciScintillaBase::SCN_MODIFIED :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::inputMethodQuery :
QsciScintillaBase::keyPressEvent :
QgsCodeEditor::keyPressEvent :
QWidget::event :
QFrame::event :
QAbstractScrollArea::event :
QApplicationPrivate::notify_helper :
QApplication::notify :
QgsApplication::notify :
QCoreApplication::notifyInternal2 :
QSizePolicy::QSizePolicy :
QApplicationPrivate::notify_helper :
QApplication::notify :
QgsApplication::notify :
QCoreApplication::notifyInternal2 :
QGuiApplicationPrivate::processKeyEvent :
QWindowSystemInterface::sendWindowSystemEvents :
QEventDispatcherWin32::processEvents :
qt_plugin_query_metadata :
QEventLoop::exec :
QDialog::exec :
QgsPropertyOverrideButton::showExpressionDialog :
QObject::qt_static_metacall :
QMenu::~QMenu :
QObject::qt_static_metacall :
QAction::activate :
QMenu::actionGeometry :
QMenu::actionGeometry :
QMenu::mouseReleaseEvent :
QWidget::event :
QMenu::event :
QApplicationPrivate::notify_helper :
QApplication::notify :
QgsApplication::notify :
QCoreApplication::notifyInternal2 :
QApplicationPrivate::sendMouseEvent :
QSizePolicy::QSizePolicy :
QSizePolicy::QSizePolicy :
QApplicationPrivate::notify_helper :
QApplication::notify :
QgsApplication::notify :
QCoreApplication::notifyInternal2 :
QGuiApplicationPrivate::processMouseEvent :
QWindowSystemInterface::sendWindowSystemEvents :
QEventDispatcherWin32::processEvents :
qt_plugin_query_metadata :
QEventLoop::exec :
QMenu::exec :
QToolButton::paintEvent :
QToolButton::showMenu :
QObject::qt_static_metacall :
QAbstractButton::clicked :
QAbstractButton::mousePressEvent :
QToolButton::mousePressEvent :
QWidget::event :
QApplicationPrivate::notify_helper :
QApplication::notify :
QgsApplication::notify :
QCoreApplication::notifyInternal2 :
QApplicationPrivate::sendMouseEvent :
QSizePolicy::QSizePolicy :
QSizePolicy::QSizePolicy :
QApplicationPrivate::notify_helper :
QApplication::notify :
QgsApplication::notify :
QCoreApplication::notifyInternal2 :
QGuiApplicationPrivate::processMouseEvent :
QWindowSystemInterface::sendWindowSystemEvents :
QEventDispatcherWin32::processEvents :
qt_plugin_query_metadata :
QEventLoop::exec :
QCoreApplication::exec :
main :
BaseThreadInitThunk :
RtlUserThreadStart :

QGIS Info
QGIS Version: 3.34.7-Prizren
QGIS code revision: 6f7d735
Compiled against Qt: 5.15.13
Running against Qt: 5.15.13
Compiled against GDAL: 3.9.0
Running against GDAL: 3.9.0

System Info
CPU Type: x86_64
Kernel Type: winnt
Kernel Version: 10.0.22631

Steps to reproduce the issue

1. choose categorized layer styling
2. click the general symbol
3. Edit layer stroke color as data defined override
4. add custom python function
5. go back to expression builder and enter new function wrapping '@layer_name' (without the apostrophes)
6. watch it burn

Versions

<style type="text/css"> p, li { white-space: pre-wrap; } </style>

QGIS version	3.34.7-Prizren	QGIS code revision	6f7d735
Qt version	5.15.13
Python version	3.12.3
GDAL/OGR version	3.9.0
PROJ version	9.4.0
EPSG Registry database version	v11.004 (2024-02-24)
GEOS version	3.12.1-CAPI-1.18.1
SQLite version	3.45.1
PDAL version	2.6.3
PostgreSQL client version	16.2
SpatiaLite version	5.1.0
QWT version	6.2.0
QScintilla2 version	2.14.1
OS version	Windows 11 Version 2009
Active Python plugins
copy_coords	0.3
mask	1.10.2
menu_from_project	2.1.0
mmqgis	2021.9.10
processing_r	4.1.0
processing_saga_nextgen	1.0.0
profiletool	4.2.6
qfieldsync	v4.9.1
QuickOSM	2.2.3
quick_layer	0.3
quick_map_services	0.19.34
rectanglify	1.0.3
splitmultipart	1.0.0
SplitPolygonShowingAreas	0.13
swissgeodownloader	2.0.1
swiss_locator	3.0.0
VoGisProfilTool	3.0.2
db_manager	0.1.20
grassprovider	2.12.99
processing	2.12.99

QGIS version 3.34.7-Prizren QGIS code revision [6f7d735](https://github.com/qgis/QGIS/commit/6f7d735c) Qt version 5.15.13 Python version 3.12.3 GDAL/OGR version 3.9.0 PROJ version 9.4.0 EPSG Registry database version v11.004 (2024-02-24) GEOS version 3.12.1-CAPI-1.18.1 SQLite version 3.45.1 PDAL version 2.6.3 PostgreSQL client version 16.2 SpatiaLite version 5.1.0 QWT version 6.2.0 QScintilla2 version 2.14.1 OS version Windows 11 Version 2009

Active Python plugins
copy_coords
0.3
mask
1.10.2
menu_from_project
2.1.0
mmqgis
2021.9.10
processing_r
4.1.0
processing_saga_nextgen
1.0.0
profiletool
4.2.6
qfieldsync
v4.9.1
QuickOSM
2.2.3
quick_layer
0.3
quick_map_services
0.19.34
rectanglify
1.0.3
splitmultipart
1.0.0
SplitPolygonShowingAreas
0.13
swissgeodownloader
2.0.1
swiss_locator
3.0.0
VoGisProfilTool
3.0.2
db_manager
0.1.20
grassprovider
2.12.99
processing
2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[nyalldawson]

This isn't a qgis bug -- you're not using the API correctly. Please use the mailing lists or stackexchange for user support.

[thymaro]

Thanks, Nyall. At least I know what the problem could be, then.