Warn if visible GraphicsView is garbage collected (#942)

* Warn if visible window is garbage collected

* (Py)Qt does not rely on Python GC

* Only warn if deleted widget has no parents (if it is a standalone window)

* Hide windows when closing

* Only implement GraphicsView.__del__ if it does not prevent circular reference garbage collection

pyqtgraph/graphicsItems/tests/test_ImageItem.py

@@ -121,7 +121,7 @@ def test_ImageItem(transpose=False):
     assertImageApproved(w, 'imageitem/resolution_with_downsampling_y', 'Resolution test with downsampling across y axis.')
     assert img._lastDownsample == (1, 4)
     
-    view.hide()
+    w.hide()
 
 def test_ImageItem_axisorder():
     # All image tests pass again using the opposite axis order

pyqtgraph/graphicsItems/tests/test_ROI.py

@@ -153,6 +153,7 @@ def check_getArrayRegion(roi, name, testResize=True, transpose=False):
     # allow the roi to be re-used
     roi.scene().removeItem(roi)
 
+    win.hide()
 
 def test_PolyLineROI():
     rois = [
@@ -234,5 +235,6 @@ def test_PolyLineROI():
         r.setState(initState)
         assertImageApproved(plt, 'roi/polylineroi/'+name+'_setstate', 'Reset ROI to initial state.')
         assert len(r.getState()['points']) == 3
-        
-    
+    
+    plt.hide()
+    

pyqtgraph/widgets/GraphicsView.py

@@ -15,6 +15,7 @@ except ImportError:
 
 from ..Point import Point
 import sys, os
+import warnings
 from .FileDialog import FileDialog
 from ..GraphicsScene import GraphicsScene
 import numpy as np
@@ -396,5 +397,11 @@ class GraphicsView(QtGui.QGraphicsView):
         
     def dragEnterEvent(self, ev):
         ev.ignore()  ## not sure why, but for some reason this class likes to consume drag events
-        
 
+    def _del(self):
+        if self.parentWidget() is None and self.isVisible():
+            msg = "Visible window deleted. To prevent this, store a reference to the window object."
+            warnings.warn(msg, RuntimeWarning, stacklevel=2)
+
+if sys.version_info[0] == 3 and sys.version_info[1] >= 4:
+    GraphicsView.__del__ = GraphicsView._del