def validPolygons(self):
'''
returns only 'valid' polygons within our list of geometric shapes
'''
previously_rejected= []
for properties in self.shape:
# create instance of shape
shape= Shape(**properties.__dict__)
# if we found a valid convex polygon yield it to our caller but no need to test twice
if shape.id not in self.previously_rejected and shape.isConvexPolygon():
yield shape
else:
# if this is a new reject display warning
if shape.id not in self.previously_rejected:
print >> stderr, shape.id, "is not a polygon"
# keep trace of which shapes we've previoulsy rejected
self.previously_rejected.append(shape.id)
