Python <function at 0x> output [duplicate]

This question already has an answer here:

• “Function ________ at 0x01D57aF0” return in python 2 answers

I wrote a new function and when I execute it, I get an error:

<function read_grades at 0x000001F69E0FC8C8>

Ok so here is my code:

def add(x, y):
    z = x / y * 100
    return z

def calc_grade(perc):
    if perc < 50:
        return "1"
    if perc <  60:
        return "2"
    if perc < 75:
        return "3"
    if perc < 90:
        return "4"
    if perc >= 90:
        return "5"

def calc_command():
    num1 = input("Input your points: ")
    num2 = input("Input maximum points: ")
    num3 = add(float(num1), float(num2))
    grade = calc_grade(num3)
    print("This is your result:", str(num3) + "%")
    print("Your grade:", grade)
    save = open("grades.txt", "r")
    read_grades = save.read()
    save = open("grades.txt", "w")
    save.write(read_grades + grade)
    save.close()

def read_grades():
    save = open("grades.txt", "r")
    read_grades = save.read()
    grades = read_grades.split()
    save.close()
    return grades

while True:
    command = input("Input your command: ")
    if command == "CALC":
        calc_command()
    elif command == "EXIT":
        break
    elif command == "GRADES":
        print(read_grades)
    elif command == "HELP":
        print("These are the commands:nCALC - Calculates your grade and writes in the file.nEXIT - Exits the program.nGRADES - Reads your previous grades.nHELP - This command. It helps you.")
    else:
        print("You inputed an invalid command. Type HELP for help.")

This error happens when I execute the read_grades() function or the GRADES command.

For those who marked this question: I did some searching and I didn't find that post and now that i read it i dont understand the answer