add docstring
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EdwardJKim/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader