What am I doing wrong?

[forumer444]

This is my updated code and it only prints out the last line in the input file which is "6464". Any ideas what I need to change? I can't use a split() function in my assignment.

# function 4
def leapYear(year):
    """ Calculates whether a year is a leap year or not. """
    year = int(year)

    return year % 4 == 0 and (year % 10 != 0 or year % 400 == 0)

# function 3
def writeFile():
    """ Opens, writes to, and closes output file. """
    file_2 = open(output_file, "w") # open output file
    file_2.write(checkRawData()) # write to output file
    file_2.close()

# function 2
def checkRawData():
    """ Checks raw data by looping through conditions. """
    for line in readFile(): # loop through input file
        if line[:4].isdigit():
            if leapYear(line[:4]):
                filtered_data = line[:4]

    return filtered_data

# function 1
def readFile():
    """ Opens, reads and closes input file. """
    try:
        file_1 = open(input_file, "r") # open input file
        lines = file_1.readlines() # read input file
        file_1.close()
    except ValueError as e:
        print(e)

    return lines

# user input
input_file = input("Enter the name of the input file you want to read: ")
output_file = input("Enter the name of the output file to which you want to write: ")

# function calls
readFile()
checkRawData()
writeFile()