reading grades from a file

def read_grades(gradefile):
    '''(file open for reading) -> list of float

    Read and return the list of grades in gradefile.
   
    Precondition: gradefile starts with a header that contains
    no blank lines, then has a blank line, and then lines
    containing a student number and a grade.
    '''

    # Skip over the header.
    line = gradefile.readline()
    while line != '\n':
        line = gradefile.readline()

    # Read the grades, accumulating them into a list.

    grades = []
    
    line = gradefile.readline()
    while line != '':
        # Now we have s string containing the info for a
        # single student.
        # Find the last space and take everything after that
        # space.
        grade = line[line.rfind(' ') + 1:]
        grades.append(float(grade))
        line = gradefile.readline()

    return grades