<｜begin▁of▁sentence｜>### Instruction:
Given a graph and a Cairo drawing context, how can you use a custom shape inheriting from the ShapeDrawer class in igraph to draw a unique vertex shape (e.g., a square) at a specified position with defined width and height, and save the resulting drawing as a PNG?
### Response:
import igraph
def draw_custom_shape(graph, context, center_x, center_y, width, height):
    # Assuming a custom shape drawer inheriting from ShapeDrawer
    class CustomShapeDrawer(igraph.drawing.shapes.ShapeDrawer):
        @staticmethod
        def draw_path(ctx, center_x, center_y, width, height, **kwargs):
            # Example custom shape: a simple square
            ctx.rectangle(center_x - width / 2, center_y - height / 2, width, height)
    # Instantiate and draw the custom shape
    drawer = CustomShapeDrawer()
    drawer.draw_path(context, center_x, center_y, width, height)

# Example usage assuming a Cairo context
import cairo
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 400, 400)
context = cairo.Context(surface)
graph = None # Placeholder, as we only demonstrate the shape drawing part
draw_custom_shape(graph, context, 100, 100, 50, 50)
surface.write_to_png('custom_shape.png')
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