Importing function to new file not working

My program is designed to create a module named MyTriangle that reads three sides of a triangle and computes the area if the input is valid. If the input is invalid, it will display "Input is invalid". The program works beautifully. The only problem is that I am trying to import module (everything except the main function) over into a different file. I have never done this before, and can't find anywhere that gives clear instructions. Here is my code:

side1, side2, side3 = eval(input("Enter three sides in a double: "))

def isValid(side1, side2, side3):

    return side1 + side2 > side3 and side1 + side3 > side2 and side2 + side3 > side1 

def area(side1, side2, side3):

    s = (side1 + side2 + side3) / 2;

    totalArea = (s * (s - side1) * (s - side2) * (s - side3)) ** 0.5    

    return totalArea

def main():  

    condition = isValid(side1, side2, side3)

    totalArea = area(side1, side2, side3)

    if condition:
        print("The area of the triangle is " + str(totalArea))
    else:
        print("input is invalid")

main()

I tried taking the "def main()" part and putting that into a file by itself. While naming the file with all the rest of the code "MyTriangle.py"

import MyTriangle
def main():  

    condition = isValid(side1, side2, side3)

    totalArea = area(side1, side2, side3)

    if condition:
        print("The area of the triangle is " + str(totalArea))
    else:
        print("input is invalid")

main()

When I run the program, it asks "Enter three sides in a double:". When I put the numbers in (for example: 1, 1, 1), it says "name 'isValid' is not defined. I am not sure if I am importing it over correctly or what. For the life of me, I cannot figure this out. A little help please?