How can I get the path of a folder selected using tkinter's askdirectory() command?

Question

How can I assign a variable to the askdirectory() command and get the full path of the selected directory?

Here's my code:

    from tkinter import*
    from tkinter import ttk
    from tkinter.filedialog import askdirectory

    root = Tk()
    root.title("Add Mods")

    mainframe = ttk.Frame(root, padding="3 3 12 12")
    mainframe.grid(column=0,row=0,sticky=(N,W,E,S))
    mainframe.columnconfigure(0, weight=1)
    mainframe.rowconfigure(0,weight=1)

    ttk.Label(mainframe, text="Select your desired installation directory, 
    and then click install").grid(column=0,row=1)
    ttk.Button(mainframe, text="Select Installation Directory", 
    command=askdirectory()).grid(column=2,row=2)

    root.mainloop()

Note: When I run the script, the function runs even though it shouldn't run until the button is pushed.

    def getDir():
        dir = askdirectory()
        return dir