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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 save_as_png(self, filename, width=300, height=250, render_time=1):
 """Open saved html file in an virtual browser and save a screen shot to PNG format."""

        self.driver.set_window_size(width, height)
        self.driver.get('file://{path}/{filename}'.format(
            path=os.getcwd(), filename=filename + ".html"))
        time.sleep(render_time)
        self.driver.save_screenshot(filename + ".png")