Why don't you use MatrixSymbol? Following Smichr's advice, your solution could look like this:

M, m = MatrixSymbol('M_m,n', 3, 3), MatrixSymbol('m', 3, 3).as_explicit()
s= '{}={}'.format(latex(M), latex(m))
display(Math(s))
s= 'diag({0})={1}, U({0})={2}, L({0})={3}'.format(latex(M),  latex(m.multiply_elementwise(eye(3))), latex(m.upper_triangular(0)), latex(m.lower_triangular(0)))
display(Math(s))