function in django settings?

I would like to add a function to my django settings.py (returning an incremented url).

However, my attempt to add this function is not working...

'Settings' object has no attribute 'nextCdnUrl'

Here is the relevant bits of the settings.py file:

CDN_MIN_I = 1
CDN_MAX_I = 6
CDN_I = CDN_MIN_I

def nextCdnUrl( ):
    CDN_I += 1
    if CDN_I > CDN_MAX_I:
        CDN_I = CDN_MIN_I

    return CDN_BASE_URL.replace( "_i", str(CDN_I) )