ValueError while trying to save a pixmap as a png file

How can I save a pixmap as a .png file?
I do this:

    image = gtk.Image()
    image.set_from_pixmap(disp.pixmap, disp.mask)
    pixbf=image.get_pixbuf()
    pixbf.save('path.png')

I get this error:

    pixbf=image.get_pixbuf()
    ValueError: image should be a GdkPixbuf or empty

From the documentation,

The get_pixbuf() method gets the gtk.gdk.Pixbuf being displayed by the gtk.Image. The return value may be None if no image data is set. If the storage type of the image is not either gtk.IMAGE_EMPTY or gtk.IMAGE_PIXBUF the ValueError exception will be raised.

(Emphasis Mine)

As you need a png file, You can follow the instructions from here

pixbuf = gtk.gdk.Pixbuf(gtk.gdk.COLORSPACE_RGB,has_alpha=False, bits_per_sample=8, width=width, height=height)
pixbuf.get_from_drawable(disp.pixmap, disp.pixmap.get_colormap(), 0, 0, 0, 0, width, height)
pixbuf.save('path.png')

This will create a pixbuf from your pixmap which is disp.pixmap. This can be later saved using pixbuf.save