user input problem

[Jerry]

In the following script I want to have the user enter the name of a text file which is later read from within a called function. It works, but a yellow error marker appears at the line that calls the function and the cursor jumps to that line. This is annoying since I then have to find the place in the script where I was editing. Can you suggest a better way to do this?

import viz

viz.go()

filename = input('filename?')

def openfile():
global filename

file = open(filename, 'r')

openfile()