GPA calculator

print( 'Welcome to the GPA calculator.' )
print( 'Please enter all your letter grades, one per line. ')
print( 'Enter a blank line to designate the end.' )
# map from letter grade to point value
points = { 'A+':4.5, 'A' :4.0, 'A-' :3.67, 'B+' :3.33, 'B' :3.0, 'B-' :2.67,
'C+' :2.33, 'C' :2.0, 'C-' :1.67, 'D+' :1.33, 'D' :1.0, 'F' :0.0}
num_courses = 0
total_points = 0
done = False
while not done:
    grade = input( ) # read line from user
    if grade =='' : # empty line was entered
        done = True
    elif grade not in points: # unrecognized grade entered
        print("Unknown grade {0} being ignored".format(grade))
    else:
        num_courses += 1
        total_points += points[grade]
if num_courses > 0: # avoid division by zero
    print( "Your GPA is {0:.3}" .format(total_points / num_courses))