code: def get_structure_seqs(pdb_file, file_type):
      
  
      # TODO: Please check out capitalization of chain IDs in mmcif files. example: 5afi - chain "l" is present but
      # it seems like biopython capitalizes it to chain L
  
      # Get the first model
      my_structure = StructureIO(pdb_file)
      model = my_structure.first_model
  
      structure_seqs = {}
  
      # Loop over each chain of the PDB
      for chain in model:
          chain_seq = ''
          tracker = 0
  
          # Loop over the residues
          for res in chain.get_residues():
              # NOTE: you can get the residue number too
              # res_num = res.id[1]
  
              # Double check if the residue name is a standard residue
              # If it is not a standard residue (ie. selenomethionine),
              # it will be filled in with an X on the next iteration)
              if Polypeptide.is_aa(res, standard=True):
                  full_id = res.get_full_id()
                  end_tracker = full_id[3][1]
                  i_code = full_id[3][2]
                  aa = Polypeptide.three_to_one(res.get_resname())
  
                  # Tracker to fill in X's
                  if end_tracker != (tracker + 1):
                      if i_code != ' ':
                          chain_seq += aa
                          tracker = end_tracker + 1
                          continue
                      else:
                          chain_seq += 'X' * (end_tracker - tracker - 1)
  
                  chain_seq += aa
                  tracker = end_tracker
  
              else:
                  continue
  
          structure_seqs[chain.get_id()] = chain_seq
  
      return structure_seqs
docs: Get a dictionary of a PDB file's sequences.
  
      Special cases include:
          - Insertion codes. In the case of residue numbers like "15A", "15B", both residues are written out. Example: 9LPR
          - HETATMs. Currently written as an "X", or unknown amino acid.
  
      Args:
          pdb_file: Path to PDB file
  
      Returns:
          dict: Dictionary of:
          {chain_id: sequence}