The line
aux=matriz;
Does not make a copy of matriz, it merely creates a new reference to matriz named aux. You probably want
aux=matriz[:]
Which will make a copy, assuming matriz is a simple data structure. If it is more complex, you should probably use copy.deepcopy
aux = copy.deepcopy(matriz)
As an aside, you don’t need semi-colons after each statement, python doesn’t use them as EOL markers.