CSV不断覆盖

我正在尝试为我的父母编写一个使用Python的快速簿记程序。我希望它如何工作如下。每次父母打开此程序时，我都希望它在excel文件中添加新行。问题在于，每次程序运行时，它都会覆盖自身。在下面，您可以看到我的代码

from tkinter import *
import csv
import pandas as pd

root = Tk()
root.title('Boekhouding 2020')
root.minsize(250, 100)
#Weeknummer
Label1 = Label(root, text= "Weeknummer").grid(row=1, column=1)
e1 = Entry(root)
e1.grid(row=1, column=2)
#Omschrijving
Label2 = Label(root, text= "Omschrijving").grid(row=2, column=1)
e2 = Entry(root)
e2.grid(row=2, column=2)
#Bedrag
Label3 = Label(root, text= "Bedrag").grid(row=3, column=1)
e3 = Entry(root)
e3.grid(row=3, column=2)



##  Exporteren naar Excel
def export(oms,bedrag,weeknummer):
    with open(r"C:\Users\frank\Desktop\Boekhouding.csv", 'w', newline='') as f:
        thewriter = csv.writer(f)
        row_export()
        thewriter.writerow([oms,bedrag,weeknummer])

## Klik definition
def myclick():
    mylabel = Label(root, text='Hello ' + e1.get())
    mylabel.grid(row=5, column=1)
    export(e2.get(),e3.get(),e1.get())

## Rowcounter
def row_export():
    with open(r"C:\Users\frank\Desktop\Boekhouding.csv",'r')as csv_file:
        fileObject = csv.reader(csv_file)
        for row in fileObject:
            print(row)
            export(row[0],row[1],row[2])



## All buttons
mybutton = Button(root, text="exporteren naar excel", command=myclick)
mybutton.grid(row=4, column=1)

root.mainloop()