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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def auto_shape_type(self):
 """Member of MSO_SHAPE indicating masking shape. A picture can be masked by any of the so-called "auto-shapes" available in PowerPoint, such as an ellipse or triangle. When a picture is masked by a shape, the shape assumes the same dimensions as the picture and the portion of the picture outside the shape boundaries does not appear. Note the default value for a newly-inserted picture is `MSO_AUTO_SHAPE_TYPE.RECTANGLE`, which performs no cropping because the extents of the rectangle exactly correspond to the extents of the picture. The available shapes correspond to the members of :ref:`MsoAutoShapeType`. The return value can also be |None|, indicating the picture either has no geometry (not expected) or has custom geometry, like a freeform shape. A picture with no geometry will have no visible representation on the slide, although it can be selected. This is because without geometry, there is no "inside-the-shape" for it to appear in. """

        prstGeom = self._pic.spPr.prstGeom
        if prstGeom is None:  # ---generally means cropped with freeform---
            return None
        return prstGeom.prst