python之预期一个缩进 block

我得到一个 Expected an indented block 这是代码，感谢您的帮助。

        #Given the menu the user will calculate the area of square, circle, rectangle 
from math import pi 
def main (): 
 
    #declare and initialize variables 
 
    #float radius = length = width = base = height = 0.0 
    radius = length = width = base = height = 0.0 
 
    #float areaCircle = areaRectangle = areaTriangle = 0.0 
    areaCircle = areaRectangle = areaTriangle = 0.0 
 
    #int menuChoice = 0 
    menuChoice = 0 
 
    #display intro 
 
    while menuChoice != 4: 
            #Display menu 
            print("Geometry Calculator") 
            print("1) Calculate the Area of a Circle") 
            print("2) Calculate the Area of a Rectangle") 
            print("3) Calculate the Area of a Triangle") 
            print("4) Quit") 
 
            #Prompt for menuChoice 
 
            if menuChoice == 1: 
                while radius < 0: 
                    #prompt for radius 
                    radius = eval(input("What is radius of circle: ")) 
                if radius < 0: 
                    #display invalid 
                    print("Invalid input. Cannot be a negative value.") 
                    #calculate areaCircle 
                    areaCircle = pi*r**2 
                    #display areaCircle 
                    print("The area of the circle is: ", areaCircle) 
 
            elif menuChoice == 2: 
                while length < 0: 
                    #prompt for length 
                    length = eval(input("What is the length of the rectangle: ")) 
                if length < 0: 
                    #display invalid 
                    print("Invalid input. Cannot be a negative value.") 
                while width < 0: 
                    #prompt for width 
                    width = eval(input("What is the width of the rectangle: ")) 
                if width < 0: 
                    #display invalid 
                    print("Invalid input. Cannot be a negative value.") 
                    #calculate areaRectangle 
                    areaRectangle = length * width 
                    #diplay areaRectangle 
                    print("The area of the rectangle is: ", areaRectangle) 
 
            elif menuChoice == 3: 
                while base < 0: 
                    #prompt for base 
                    base = eval(input("What is the length of the base of the triangle:")) 
                if base < 0: 
                   #display invalid 
                    print("Invalid input. Cannot be a negative value.") 
                while height < 0: 
                    #prompt for height 
                    height = eval(input("What is height of triangle")) 
                if height < 0: 
                    #display invalid 
                    print("Invalid input. Cannot be a negative value.") 
                    #calculate areaTriangle 
                    areaTriangle = 1/2 * base * height 
                    #display areaTriangle 
                    print("The area of the triangle is: ", areaTriangle) 
 
            elif menuChoice == 4: 
                #display exit message 
 
            else: 
                #display invalid 
                print("You must choose a number between 1-4 from the menu") 

错误在else处弹出。我试过一次缩进一个，这可能是我在编程第三周时忽略的一些小问题。