Python Error str' object is not callable : My code is:

I don't understand how I am getting this error please can someone help

import time
import os
import xlwt
from datetime import datetime
num = 0



def default():
    global num
    global model
    global partnum
    global serialnum
    global countryorigin
    
    time.sleep(1)
    print ("Model: ")
    model = input()
    print ()
    print ("Part number: ")
    partnum = input()
    print()
    print ("Serial Number: ")
    serialnum = input()
    print ()
    print ("Country of origin: ")
    countryorigin = input()
    print ("Thanks")
    num = num+1
    xlwt()
def xlwt():
    print ("Do you want to write to excel?")
    excel = input()
    if excel == "y" or "yes":
        excel()
    else:
        print ("Bye")
        sys.exit()

def excel():
    print ("Enter a spreadsheet name")
    name = input()
    wb = xlwt.Workbook()
    ws = wb.add_sheet(name)

    ws.write(0,0,"Model")
    ws.write(0,1,"Part Number")
    ws.write(0,2,"Serial Number")
    ws.write(0,3,"Country Of Origin")

    ws.write(num,0,model)
    ws.write(num,1,partnum)
    ws.write(num,2,serialnum)
    ws.write(num,3,countryorigin)

    ws.save(name)



def custom():
    print()
    
    

def main():
    print ("Welcome")
    print ()
    print ("The deafult catagories are: Model, Part Number, Serial Number,"
           "country of origin")
    time.sleep(1)
    print()
    dorc()
def dorc():    
    print ("Would you like to use the default or custom?")
    dorc = input ()
    if dorc == "default":
        default()
    elif dorc == "custom":
        custom()
    else:
        print ("Invalid input")
        dorc()


main()
           
