I think the cmp operator on two fonts is not working correctly … This works:

sf = CurrentFont()
print "Current:", sf

for f in AllFonts():
    if f == sf:
        print "equal:", f
    else:
        print "not equal:", f

result:

Current: Font Light
equal: Font Light
not equal: Font Bold
not equal: Font Book

but this gives the opposite of the expected result:

sf = CurrentFont()
print "Current:", sf

for f in AllFonts():
    if f != sf:
        print "not equal:", f
    else:
        print "equal:", f

result:

Current: Font Light
not equal: Font Light
equal: Font Bold
equal: Font Book