openpyxl KeyError Worksheet {0} does not exist

I know you probably have seen this title, but please dont mark as duplicate as my error is different. I am making an excel comparing program with openpyxl and using tkinter to make the UI more friendly. Here is my code:

import openpyxl
from tkinter import *
from tkinter.filedialog import askopenfilename
from openpyxl.utils import get_column_letter, column_index_from_string

f1 = ''
f2 = ''
sheet1 = ''
sheet2 = ''

root = Tk()
root.configure(background='light green')
root.geometry("500x500")
root.wm_title("BananaCell")

e1 = Text(root, width=15, height=1)
e1.pack()
e1.place(x=70, y=150)


e2 = Text(root, width=15, height=1)
e2.pack()
e2.place(x=300, y=150)

def destroy():
    root.destroy()

def get_1():
    inputValue = e1.get("1.0", "end-1c")
    print(inputValue)

def get_2():
    inputValue2 = e2.get("1.0", "end-1c")
    print(inputValue2)

bf = Button(root, text="Enter", width=6, height=0, command=get_1)
bf.pack()
bf.place(x=15, y=147)

bf = Button(root, text="Enter", width=6, height=0, command=get_2)
bf.pack()
bf.place(x=430, y=147)

def askForFileName1():
    global f1
    f1 = askopenfilename(title="Select Workbook 1")
    print(str(f1))


def askForFileName2():
    global f2
    f2 = askopenfilename(title="Select Workbook 2")
    print(str(f2))

sheet1 = e1.get("1.0", "end-1c")
sheet2 = e2.get("1.0", "end-1c")

b = Button(root, text="Workbook 1", width=12, height=2, command=askForFileName1)
b.pack()
b.place(x=100, y=100)

b2 = Button(root, text="Workbook 2", width=12, height=2, command=askForFileName2)
b2.pack()
b2.place(x=300, y=100)

mainloop()

wb = openpyxl.load_workbook(str(f1))
wb1 = openpyxl.load_workbook(str(f2))

ws = wb.get_sheet_by_name(str(sheet1))
ws1 = wb1.get_sheet_by_name(str(sheet2))

col1 = input('Column letter from Sheet 1 to compare from: ')
col2 = input('Column letter  from Sheet 2 to compare from: ')

for (col, col_1) in zip(ws.iter_cols(min_col = column_index_from_string(col1), max_col=column_index_from_string(col1)), ws1.iter_cols(min_col = column_index_from_string(col2), max_col=column_index_from_string(col2))):
    for (cell, cell_1) in zip(col, col_1):
       if cell.value != cell_1.value and cell.row == cell_1.row:
           print('Row ' + str(cell.row) + ' ' + str(cell.value) + ' is not 
equal to ' + str(cell_1.value) + ' ' + 'Row ' + str(cell_1.row))

exit_if = input('Press x to exit when you\'re ready: ')

Everything runs fine until I get to the line with: ws = wb.get_sheet_by_name(str(sheet1)). After that line it gives me the error:

raise KeyError("Worksheet {0} does not exist.".format(key))
KeyError: 'Worksheet  does not exist.'

Can anyone fix this? any help would be much appreciated.