Syntax Error: Invalid Syntax in a while loop

[sydney]

I'm fairly new to this stuff and keep getting an error message on line 19:

(while user_input.lower() != "quit":
^
SyntaxError: invalid syntax

let me know if you see the problem! :)

# import math library
import math
# import volume.py to import the volume calculations
import volume
# define main function in order to store the given variables
def main():
    cube_vol = 0
    pyramid_vol = 0
    ellip_vol = 0
    cube_result = []
    pyramid_result = []
    ellipsoid_result = []
    option = ""
    user_input = (input("Choose a shape: cube, pyramid, or ellipsoid or q/quit to quit")

    while user_input.lower() != "quit":
    # if the user inputs "cube", the following path is executed to prompt the user with instructions to determine the volume
        if user_input.lower() == "cube":
            # program will ask user to enter the cube's width
            cube_width = input("Enter cube width:")
             # if the input is valid, the program will calculate the volume of the cube
            if isDigit(cube_width) == True:
            cube_vol = cube_volume(float(cube_width))
            #use imported values from volume file to determine the volume of the specific cube value
            volume.cube_volume(cube_vol)
            print("The volume of the cube with the length of {} is {}.".format(width, cube_vol))
            # if the output was invalid (not a valid number) then the program will display the input is invalid
            else:
            option = "cube"
            print("Invalid input. Please try again.")

[jefsummers]

Please use Python tags for formatting code in your posts.
That said, in the line before that you have mismatched parentheses - 2 open and only 1 close.