enables functionality of holding SHIFT to draw circle or square

src/biaplotter/selectors.py

@@ -7,6 +7,8 @@
 from abc import ABC, abstractmethod
 from matplotlib.path import Path as mplPath
 from matplotlib.widgets import LassoSelector, RectangleSelector, EllipseSelector
+from qtpy.QtGui import QGuiApplication
+from qtpy.QtCore import Qt
 
 
 if TYPE_CHECKING:
@@ -64,6 +66,23 @@
             self._selector = None
 
 
+class MplRectangleSelector(RectangleSelector):
+    """Custom rectangle selector class.
+
+    Sub-class of matplotlib RectangleSelector to ensure, via 'qtpy', the option to draw a square when holding the SHIFT key.
+
+    Note: matplotlib RectangleSelector already has this functionality via the 'state_modifier_keys' argument, but it doesn't work if the canvas is used inside napari.
+    """
+    def _onmove(self, event):
+        modifiers = QGuiApplication.keyboardModifiers()
+        if modifiers == Qt.ShiftModifier:
+           self.add_state('square')
+        else:
+            if 'square' in self._state:
+                self._state.remove('square')
+        super()._onmove(event)
+
+
 class BaseRectangleSelector(Selector):
     """Base class for creating a rectangle selector.	
 
@@ -131,11 +150,28 @@
         Interactive is set to True to allow for interaction.
         Drag from anywhere is set to True to allow for drawing from any point.
         """
-        self._selector = RectangleSelector(self.ax, self.on_select, useblit=True, button=[
+        self._selector = MplRectangleSelector(self.ax, self.on_select, useblit=True, button=[
                                            1], minspanx=5, minspany=5, spancoords='pixels', interactive=True, drag_from_anywhere=True,
                                            props=dict(facecolor='#00c18c', edgecolor='#00c18c', alpha=0.3, fill=True, linewidth=2.5, linestyle='--'))
 
 
+class MplEllipseSelector(EllipseSelector):
+    """Custom ellipse selector class.
+
+    Sub-class of matplotlib EllipseSelector to ensure, via 'qtpy', the option to draw a circle when holding the SHIFT key.
+
+    Note: matplotlib EllipeseSelector already has this functionality via the 'state_modifier_keys' argument, but it doesn't work if the canvas is used inside napari.
+    """
+    def _onmove(self, event):
+        modifiers = QGuiApplication.keyboardModifiers()
+        if modifiers == Qt.ShiftModifier:
+           self.add_state('square')
+        else:
+            if 'square' in self._state:
+                self._state.remove('square')
+        super()._onmove(event)
+
+
 class BaseEllipseSelector(Selector):
     """Base class for creating an ellipse selector.
 
@@ -204,7 +240,7 @@
         Interactive is set to True to allow for interaction.
         Drag from anywhere is set to True to allow for drawing from any point.
         """
-        self._selector = EllipseSelector(self.ax, self.on_select, useblit=True, button=[
+        self._selector = MplEllipseSelector(self.ax, self.on_select, useblit=True, button=[
             1], minspanx=5, minspany=5, spancoords='pixels', interactive=True, drag_from_anywhere=True,
             props=dict(facecolor='#00c18c', edgecolor='#00c18c', alpha=0.3, fill=True, linewidth=2.5, linestyle='--'))