while cell_pos.value:

while cell_pos.value and cell_alt.value and cell_ref.value:

while cell_pos.value or cell_alt.value or cell_ref.value:

#!/usr/bin/python
import os
import csv
from openpyxl import load_workbook
from datetime import datetime
from collections import namedtuple

# Variables
sheet_directory = '<absolute_path_till_sheet_directory>'
txt_file = '<absolute_path_till_text_directory>/scores.txt'

def process_xl_sheets():
    # Process the text file and form the dictionary of positions
    dict_pos = {}
    Scores = namedtuple("Scores", ["POS", "ALT", "REF"])
    first_line = True
    with open(txt_file) as txt_filename:
        for line in txt_filename:
            if not line.strip():   # Skip empty lines
                continue
            if first_line:         # Skip the header
                first_line = False
                continue
            line = line.rstrip('\n')
            x = line.split('\t')
            cpos = Scores(POS=x[0], ALT=x[2], REF=x[1])
            dict_pos[cpos] = x[3]

    # Now process all Excel files
    pos_col_no = 'E'
    alt_col_no = 'G'
    ref_col_no = 'F'
    score_col_no = 'V'
    row_no = 4
    for sheet_root, sheet_dirs, sheet_files in os.walk(sheet_directory):
        for sheet_file in sheet_files:
            if sheet_file.endswith('.xlsx'):
                sheet_xl_file = os.path.join(sheet_root, sheet_file)
                wb = load_workbook(sheet_xl_file, data_only=True)
                ws = wb.get_sheet_by_name('raw_data')
                pos = ws[pos_col_no + str(row_no)].value
                alt = ws[alt_col_no + str(row_no)].value
                ref = ws[ref_col_no + str(row_no)].value
                while pos or alt or ref:
                    cpos = Scores(POS=str(pos), ALT=alt, REF=ref)
                    if cpos in dict_pos:
                        ws[score_col_no + str(row_no)].value = dict_pos[cpos]
                    else:
                        ws[score_col_no + str(row_no)].value = 'Unknown_' + datetime.now().strftime("%B") + datetime.now().strftime("%Y")
                    row_no += 1
                    pos = ws[pos_col_no + str(row_no)].value
                    alt = ws[alt_col_no + str(row_no)].value
                    ref = ws[ref_col_no + str(row_no)].value
    wb.save(sheet_xl_file)

# Main section
process_xl_sheets()