Why is QColorDialog.getColor crashing unexpectedly?

Question

I got this little mcve code:

import sys
import re

from PyQt5 import QtGui, QtWidgets, QtCore
from PyQt5.Qsci import QsciScintilla
from PyQt5 import Qsci


class SimpleEditor(QsciScintilla):

    def __init__(self, language=None, parent=None):
        super().__init__(parent)

        font = QtGui.QFont()
        font.setFamily('Courier')
        font.setFixedPitch(True)
        font.setPointSize(10)
        self.setFont(font)
        self.setMarginsFont(font)
        fontmetrics = QtGui.QFontMetrics(font)
        self.setMarginsFont(font)
        self.setMarginWidth(0, fontmetrics.width("00000") + 6)
        self.setMarginLineNumbers(0, True)
        self.setMarginsBackgroundColor(QtGui.QColor("#cccccc"))
        self.setBraceMatching(QsciScintilla.SloppyBraceMatch)
        self.setCaretLineVisible(True)
        self.setCaretLineBackgroundColor(QtGui.QColor("#E8E8FF"))

        if language:
            self.lexer = getattr(Qsci, 'QsciLexer' + language)()
            self.setLexer(self.lexer)

        self.SendScintilla(QsciScintilla.SCI_FOLDALL, True)
        self.setAutoCompletionThreshold(1)
        self.setAutoCompletionSource(QsciScintilla.AcsAPIs)
        self.setFolding(QsciScintilla.BoxedTreeFoldStyle)

        # Signals/Slots
        self.cursorPositionChanged.connect(self.on_cursor_position_changed)
        self.copyAvailable.connect(self.on_copy_available)
        self.indicatorClicked.connect(self.on_indicator_clicked)
        self.indicatorReleased.connect(self.on_indicator_released)
        self.linesChanged.connect(self.on_lines_changed)
        self.marginClicked.connect(self.on_margin_clicked)
        self.modificationAttempted.connect(self.on_modification_attempted)
        self.modificationChanged.connect(self.on_modification_changed)
        self.selectionChanged.connect(self.on_selection_changed)
        self.textChanged.connect(self.on_text_changed)
        self.userListActivated.connect(self.on_user_list_activated)

    def on_cursor_position_changed(self, line, index):
        text = self.text(line)
        for match in re.finditer('(?:^|(?<=\W))\d+(?:\.\d+)?(?=$|\W)', text):
            start, end = match.span()
            if start <= index <= end:
                pos = self.positionFromLineIndex(line, start)
                x = self.SendScintilla(
                    QsciScintilla.SCI_POINTXFROMPOSITION, 0, pos)
                y = self.SendScintilla(
                    QsciScintilla.SCI_POINTYFROMPOSITION, 0, pos)
                point = self.mapToGlobal(QtCore.QPoint(x, y))
                num = float(match.group())
                message = 'number: %s' % num
                break
        else:
            point = QtCore.QPoint(0, 0)
            message = ''
        QtWidgets.QToolTip.showText(point, message)
        color = QtWidgets.QColorDialog.getColor()

    def on_copy_available(self, yes):
        print('-' * 80)
        print("on_copy_available")

    def on_indicator_clicked(self, line, index, state):
        print("on_indicator_clicked")

    def on_indicator_released(self, line, index, state):
        print("on_indicator_released")

    def on_lines_changed(self):
        print("on_lines_changed")

    def on_margin_clicked(self, margin, line, state):
        print("on_margin_clicked")

    def on_modification_attempted(self):
        print("on_modification_attempted")

    def on_modification_changed(self):
        print("on_modification_changed")

    def on_selection_changed(self):
        print("on_selection_changed")

    def on_text_changed(self):
        print("on_text_changed")

    def on_user_list_activated(self, id, text):
        print("on_user_list_activated")


def show_requirements():
    print(sys.version)
    print(QtCore.QT_VERSION_STR)
    print(QtCore.PYQT_VERSION_STR)

if __name__ == "__main__":
    show_requirements()

    app = QtWidgets.QApplication(sys.argv)

    ex = QtWidgets.QWidget()
    hlayout = QtWidgets.QHBoxLayout()
    ed = SimpleEditor("JavaScript")

    hlayout.addWidget(ed)

    ed.setText("""#ifdef GL_ES
precision mediump float;
#endif

#extension GL_OES_standard_derivatives : enable

uniform float time;
uniform vec2 mouse;
uniform vec2 resolution;

void main( void ) {

    vec2 st = ( gl_FragCoord.xy / resolution.xy );
    vec2 lefbot = step(vec2(0.1), st);
    float pct = lefbot.x*lefbot.y;
    vec2 rigtop = step(vec2(0.1), 1.-st);
    pct *= rigtop.x*rigtop.y;
    vec3 color = vec3(pct);

    gl_FragColor = vec4( color, 1.0 );""")

    ex.setLayout(hlayout)
    ex.show()
    ex.resize(800, 600)

    sys.exit(app.exec_())

For some unknown reason, when I press OK after picking up a color the script will crash unexpectedly, here's a video showing what I mean.

Any idea why this would happen?

Answer 1

Opening a dialog with a blocking exec() inside the cursor-change handler seems like a bad idea, so I'm not surprised it crashes. It will be much safer to open the dialog asynchronously and use a signal/slot to handle the result. Using a closure with show() would seem best, as it will give access to the current local variables:

class SimpleEditor(QsciScintilla):
    def __init__(self, language=None, parent=None):
        super().__init__(parent)
        ...
        self.colors = QtWidgets.QColorDialog(self)

    def on_cursor_position_changed(self, line, index):
        text = self.text(line)
        for match in re.finditer('(?:^|(?<=\W))\d+(?:\.\d+)?(?=$|\W)', text):
            start, end = match.span()
            if start <= index <= end:
                pos = self.positionFromLineIndex(line, start)
                x = self.SendScintilla(
                    QsciScintilla.SCI_POINTXFROMPOSITION, 0, pos)
                y = self.SendScintilla(
                    QsciScintilla.SCI_POINTYFROMPOSITION, 0, pos)
                point = self.mapToGlobal(QtCore.QPoint(x, y))
                num = float(match.group())
                message = 'number: %s' % num

                def handler():
                    print(message)
                    print(self.colors.selectedColor().name())
                    self.colors.finished.disconnect()

                self.colors.finished.connect(handler)
                self.colors.show()

                break