residue = structure[0]['A'][100]

residue = structure[0]['A'][2003]

File "<stdin>", line 1, in <module>

File "/home/azevedo/.local/lib/python3.5/site-packages/Bio/PDB/Chain.py", line 94, in __getitem__
return Entity.__getitem__(self, id)

File "/home/azevedo/.local/lib/python3.5/site-packages/Bio/PDB/Entity.py", line 41, in __getitem__
return self.child_dict[id]

structure[0]['A'][('H_CYS', 2003, ' ')]

<Residue CYS het=H_CYS resseq=2003 icode= >

def get_residue_by_number(residues, number):
    for residue in residues:
        if residue.id[1] == number:
            return residue

get_residue_by_number(structure[0]['A'].get_residues(), 2003)

<Residue CYS het=H_CYS resseq=2003 icode= >

get_residue_by_number(structure[0]['A'].get_residues(), 100)

<Residue ASP het=  resseq=100 icode= >