A FreeCAD script for the purpose of inspecting the radius and center of circles in 3D models

MeasureCircle.py

from FreeCAD import Base

class MeasureCircle:
    """This class will report the computed radius and center of a circle or arc given 3 vertices.
        A line is drawn from the first vertex to the center of the circle, which may be used for future measurements.
        Just select the vertices and the result will be shown in Report View
        Edges may also be selected and count as two vertices.  
        If two edges are selected one of the vertices in the second edge is not used in the calculation"""
    def __init__(self):
        self.points = []
        Gui.Selection.addObserver(self)
        FreeCAD.Console.PrintMessage("Select any 3 vertices on a circle\n")

    # don't allow duplicate points
    def addPoint(self, v):
        for p in self.points:
            if p.Point == v.Point:
                return
        self.points.append(v)

    def addSelection(self, doc, obj, sub, pos):
        o = App.getDocument(doc).getObject(obj)
        if sub != '' and type(o) == Part.Feature:
            e = getattr(o.Shape, sub)
            if e.ShapeType == 'Edge':
                self.addPoint(e.Vertexes[0])
                self.addPoint(e.Vertexes[1])
            elif e.ShapeType == 'Vertex':
                self.addPoint(e)
            if len(self.points) >= 3:
                Gui.Selection.removeObserver(self) 
                v1 = self.points[0].Point
                v2 = self.points[1].Point
                v3 = self.points[2].Point
                curve = Part.Arc(v1, v2, v3).toShape().Curve
                line = Part.makeLine(v1, curve.Center)
                obj = FreeCAD.ActiveDocument.addObject("Part::Feature","Radius")
                obj.Shape = line
                FreeCAD.Console.PrintMessage("Radius: %f\n" % curve.Radius)
                FreeCAD.Console.PrintMessage("Center: X=%f, Y=%f, Z=%f\n" % (curve.Center.x, curve.Center.y, curve.Center.z))

if __name__ == "__main__":
    MeasureCircle()