I think you will not get a midpoint if the line is irregular.
Actually the truecentroid property will give you a gravity center of an irregular polyline but if you want to reach the middle point of a feature you have to do something else.
The picture above is wrong. The centroid property will give you the nearest point from centroid (actually, within the polyline).
Midpoint = geometry.positionAlongLine(0.50,True).firstPoint... print Midpoint.X... print Midpoint.Y
I think this will be the runtime equivalent method then: GeometryEngine.CreatePointAlong Method
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