在函数中调用参数（Python）

Question

I am new to python and am just trying to make my first program to compute what someone needs to get on their final to obtain their target grade (similar to rogerhub http://rogerhub.com/final-grade-calculator/ ). I have a formula that works but cannot seem to call the arguments within the function without running into an error. As I said, I just started learning python so I am sure this is just a simple mistake. Any help would be greatly appreciated.

current_grade = float(raw_input("Your current grade is:"))
target_grade = float(raw_input("You want at least a :"))
final_percentage = float(raw_input("Your Final is worth:"))
final_percentage = (final_percentage / 100)
semester_percentage = 1 - (final_percentage)


def class_grade(current_grade, target_grade, final_percentage):
    grade_needed = ((target_grade - current_grade * (1 - final_percentage))/ final_percentage)
    return grade_needed

def main():
    pass

if __name__ == '__main__':
    main()

class_grade(current_grade, target_grade, final_percentage)

print "In order go get %s your need to get a %s on your final" % (target_grade, class_grade)

""" Formula for program

Insert your known numbers into the following formula:
G= (D-C(1-P))/P

G is the grade you will need on the final test.
C is the grade you're currently carrying in the class.
D stands for what your school has determined as a passing grade. This should be 60 or 70.
P is the percentage of the total grade for which the final test counts.
Both C and D should be whole numbers, while a decimal should be used for P.
"""

Answer 1

class_grade is a function. To make it run, you have to put it's arguments behind it in parentheses:

class_grade(current_grade, target_grade, final_percentage)

The end result:

print "In order go get %s your need to get a %s on your final" % (target_grade, class_grade(current_grade, target_grade, final_percentage))