How to select only RNA with Hetero atoms from pdb file with python?
Solution I found: (c/p of body)

class RNASelect(Select):
    def accept_residue(self, residue):
        return 1 if ((residue.id[0] != 'H_ MG') and ((residue.get_resname() == '  A' or residue.get_resname() == '  U' or   residue.get_resname() == '  G' or  residue.get_resname() == '  C' or residue.get_resname() == '  T') or  (residue.id[0][0:2] == 'H_' and (residue.get_resname()[2] == 'U'  or residue.get_resname()[2] == 'A' or residue.get_resname()[2] == 'C' or residue.get_resname()[2] == 'G' or residue.get_resname()[2] == 'T'))))  else 0


It's not pretty but it do the job. The idea is to get everything that finish by a nucleotide. I figured out that hetatom's names of RNA chains always finish by a nucleotide letter so i just have to check everything that begins by 'H_' and ends by 'A,T,C,G,U'. Just remove 'H_ MG' and you're good.