multiline.py

# initializing variables
transactions = []                 # list to store single row entries
d_i = index of date column        # from BANK_DETAILS
p_i = index of particular column  # from BANK_DETAILS

# iterate over all transactions
for v_t in valid_transactions:
    # date exists in the row
    if v_t[d_i] is not None:
        transactions.append(v_t)
    
    # date empty, multiline particular
    else:
        # add v_t's particular to last entry in transactions
        transactions[-1][p_i] += v_t[p_i]