Python, tk: How do I prevent the execfile from running automatically?

Question

So I'm trying to make a main menu. But my code automatically runs execfile("list.py") before making the button for it.

def list():
    master.withdraw()
    execfile("list.py")

listb = Button(master, text = "Patient List", command=list())
listb.grid(row=1, column=1)

How do I fix this?