Here's a Boolean thing:
if (not suffix == "flac" ) or (not suffix == "cue" ): # WRONG! FAILS
print filename + ' is not a flac or cue file'
but
if not (suffix == "flac" or suffix == "cue" ): # CORRECT!
print filename + ' is not a flac or cue file'
(not a) or (not b) == not ( a and b )
,
is false only if a and b are both true
not (a or b)
is true only if a and be are both false.