我在我的python代码上收到以下错误“NameError：name'ref_atoms'未定义”

Question

我想知道为什么我为以下代码收到此错误，因为我已将ref_atoms定义为['CA']。错误发生在第121行，其中为下面的代码行super_imposer.set_atoms(ref_atoms, sample_atoms)

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def alignPDB(potentialTag,tagProtein):

    # Select what residues numbers you wish to align
    # and put them in a list
    start_id = 1
    end_id   = 10
    atoms_to_be_aligned = range(start_id, end_id + 1)

    # Start the parser
    pdb_parser = Bio.PDB.PDBParser(QUIET = True)

    # Get the structures
    ref_structure = pdb_parser.get_structure("tagProtein", "4ABN.pdb")
    sample_structure = pdb_parser.get_structure("potentialTag", "2LYZ.pdb")

    # Use the first model in the pdb-files for alignment
    # Change the number 0 if you want to align to another structure
    ref_model    = ref_structure[0]
    sample_model = sample_structure[0]

    # Make a list of the atoms (in the structures) you wish to align.
    # In this case we use CA atoms whose index is in the specified range
    ref_atoms = ['CA']
    sample_atoms = ['CA']

    # Iterate of all chains in the model in order to find all residues
    for ref_chain in ref_model:
    # Iterate of all residues in each model in order to find proper atoms
        for ref_res in ref_chain:
    # Check if residue number ( .get_id() ) is in the list
            if ref_res.get_id()[1] in atoms_to_be_aligned:
        # Append CA atom to list
                ref_atoms.append(ref_res['CA'])

    # Do the same for the sample structure
    for sample_chain in sample_model:
        for sample_res in sample_chain:
            if sample_res.get_id()[1] in atoms_to_be_aligned:
                sample_atoms.append(sample_res['CA'])

    # Now we initiate the superimposer:
    super_imposer = Bio.PDB.Superimposer()
    super_imposer.set_atoms(ref_atoms, sample_atoms)
    super_imposer.apply(sample_model.get_atoms())

    # Print RMSD:
    print super_imposer.rms

    # Save the aligned version of 1UBQ.pdb
    io = Bio.PDB.PDBIO()
    io.set_structure(sample_structure)
    io.save("2LYZ_aligned.pdb")