except as a control flow mechanism

#quote from MIT 'introduction to computation and programming using python, Revised' def getGrades(fname):    try:        gradesFile = open(fname, 'r') #open file for reading    except:        raise ValueError('getGrades could not open ' + fname)    grades = []    for line in gradesFile:        try:            grades.append(float(line))        except:            raise ValueError('Unable to convert line to float')    return grades    try:    grades = getGrades('quiz.txt')    grades.sort()    median = grades[len(grades)//2]    print 'Median grade is', medianexcept ValueError, errorMsg:    print 'Whoops.', errorMsg