List variable changing to

Hi I am trying to read precipitation files from a folder and calculate total precipitation for a year. Using os and pandas to loop through sub director and read files. Each file contains data for 1 year. Then calculating total precipitation for each year.

I am also trying to get a list of years where total precipitation is zero. So I declared an empty list (zero_year = []) then used if condition to append years where total precipitation is zero to the empty list.

My problem is that the zero_year switches to class 'NoneType' inside the if statement and will not allow me to append the years with zero precipitation to zero_year list.

I think I am missing something very basic. Appreciate any help I can get.

#Walk through all subfolders and files under the "Precipitation" directory
#1st loop gets list of sub directory under precipitation
#2nd loop is for files under the sub directory
import os
import pandas as pd
filepath = "Precipitation\\"

global zero_year
for sub_directory in os.listdir(filepath):
    sub_filepath = (filepath+sub_directory)
   
    zero_year = []
    print(type(zero_year))

    for filename in os.listdir(sub_filepath):
        full_filepath = (sub_filepath+ "\\" +filename)
        current_year = filename[6:8]+ filename[9:]
                        
        #Read Precipitation File
        precip_file = pd.read_csv(full_filepath,sep = " ",header = None,skipinitialspace = True,names = ["Day","Precip (mm)"],index_col=False)
        
        #Remove the last row that has no data using drop
        precip_file.drop(precip_file.tail(1).index,inplace=True)

        #Convert all strings to no data ---- NaN
        precip_file =precip_file.apply(pd.to_numeric,errors = "coerce")
        
        #Sum all precipitation value in file to get total precipitation value for a year
        yearly_total = precip_file["Precip (mm)"].sum(skipna=True)
          
        #List years where total precipitation is zero
        if yearly_total == 0:                     
            zero_year = zero_year.extend(current_year)
            print(type(zero_year))
            print(zero_year)