How do I call a file imported through tkinter filedialog?

I am trying to create a very simple GUI that will import a file, run it through some data formatting code, and export it as an .xlsx file

The file would be an excel file. An example would be:

col1 
a        
b
c

and my current python script does this:

df = read_excel('file.xlsx')
mapping = {'a':'apple','b':'banana','c':'carrot'}
df = df.replace({"col1":mapping}, regex=True)

and it returns:

col1
apple
banana
carrot

but now I am trying to create a GUI that will run it instead (:

This is the code I have so far (I get the error ValueError: DataFrame constructor not properly called!):

import tkinter as tk
from tkinter import filedialog
import pandas as pd 
from datetime import datetime, date

def UploadAction(event=None):
    filename = filedialog.askopenfilename()

    print('Selected:', filename)

    df = pd.DataFrame(eval(data=filename))

    mmapping = {'a':'apple','b':'banana','c':'carrot'}

    df = df.replace({"col1":mapping}, regex=True)

    print(df['col1'])

root = tk.Tk()
button = tk.Button(root, text='Open', command=UploadAction)
button.pack()

root.mainloop()

For the excel export, I know the code should be:

writer = pd.ExcelWriter("newfile.xlsx",
                        engine='xlsxwriter',
                        datetime_format='yyyymmdd',
                        date_format='yyyymmdd')

df.to_excel(writer, sheet_name = ('Sheet1'))

workbook  = writer.book
worksheet = writer.sheets['Sheet1']
worksheet.set_column('B:C', 20)
writer.save()

but I am not sure of how to include it in the GUI program.

So what would you suggest?