A Simple GUI Amortization Program
Simple? Really?
After posting the bare bones function for amortization – ( see:
OTHER MODULES & Stuff ->
Spreadsheet formulas: a Non-Module ->
An easy amortization Schedule Function )
…which is only 36 lines long, I decided to take a few minutes and stuff it into a GUI frame to demo a few other issues with special thought to showing how easy the ttk Combobox makes selections from a list.

That was 5 weeks and about a 1000 hours of work and research ago.

The combo boxes took about 5 minutes. The user error checking, and associated issues, took the 5 weeks.

Of course, I am just a hobbyist and part time programmer with a lot going on in real life. Still, I never thought this would take more than a couple of hours or that I would have to build, from scratch, input vetting routines. Blogs on the issues will follow, but for now here it is, and the numbers it returns have been compared to those generated by one of America’s major mortgage companies with results within one cent.

# Project: Amortization Table in GUI
# Author: John A. Oakey   (c) 2018

# Header Section: GUI basic project set up - creates a 1024x768 work area on a full screen background
from tkinter import *
from tkinter.ttk import *  # added just for this app
from tkinter import messagebox
import math
root = Tk()
root.attributes('-fullscreen', True)
root.configure(background='SteelBlue4')
scrW = root.winfo_screenwidth()
scrH = root.winfo_screenheight()  
workwindow = str(1024) + "x" + str(768) + "+" + str(int((scrW-1024)/2)) + "+" + str(int((scrH-768)/2))
top1 = Toplevel(root, bg="light blue")
top1.geometry(workwindow)
top1.title(" Simple Amortization")
top1.attributes("-topmost", 1)  # make sure top1 is on top to start - "wax on"
root.update()                   # but don't leave it locked in place
top1.attributes("-topmost", 0)  # in case you use lower or lift  -  "wax off"
# exit button - note: uses grid
b3 = Button(root, text="Egress", command=root.destroy)
b3.grid(row=0, column=0, ipadx=10, ipady=10, pady=5, padx=5, sticky=W+N)


# ____________________________

# Section I: Constants, variables, etc
setup = False
loan_value = StringVar()
loan_value.set("100.01")
apr_input = StringVar()
apr_input.set("3.5")
apr = StringVar()
apr.set(apr_input.get())
rper = (3.5/100)/12
payment = StringVar()
payment.set('10.01')
new_balance = StringVar()
new_balance.set(1)
start_month = StringVar()
start_month.set("Month")
start_yr = StringVar()
start_yr.set('Year')
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
years = []
for year in range(1980, 2040):
    years.append(str(year))
date_label_header = "Select MONTH and YEAR to begin your mortgage.\nFor example 'Jan' for a month and '2015' for a year.\n(Available years between 1980 and 2040.)"
focOutWid = StringVar()
focOutWid.set("loan")
focOutWidVal = StringVar()
focOutWidVal.set('100000.01')

# ____________________________

# Section II: Functions

# (1) Utility Functions


def zap_commas(num):
        if "," in num:                     # some users insist on using commas
            num = "".join(num.split(","))  # so remove them and continue checking
        return num


def number_check(num, _min, _max):  # need to check several inputs so use this function
    try:
        numchk = float(num)
    except ValueError:
        return False
    else:
        if _min <= numchk <= _max:
            return True
        else:
            return False


def bad_entry_msg(entry_type, entry_widget, reset_amt_string):
    mymessage = "Oops, Invalid Entry for " + entry_type
    messagebox.showinfo("Incorrect Data Emtry", mymessage, parent=data_frame)
    eval(reset_amt_string)
    eval(entry_widget + '.focus_set()')
    eval(entry_widget + '.selection_range(0, END)')
    return "break"


def badnews(msg):
    messagebox.showinfo("Data Error", msg, parent=data_frame)
    loan_value.set("100.01")
    payment.set('10.01')
    # apr.set('3.5')
    loan_entry.focus_set()
    loan_entry.selection_range(0, END)
    return "break"


def clickcheck(self):
    # if user left clicks in an entry and moves focus with the mouse, bad entry info can be uncorrected
    test = loan_amount()
    if test == "break":
        loan_entry.focus_set()
        loan_entry.selection_range(0, END)
        return "break"
    test = pay_amount()
    if test == "break":
        payment_entry.focus_set()
        payment_entry.selection_range(0, END)
        return "break"
    test = apr_amount()
    if test == "break":
        apr_entry.focus_set()
        apr_entry.selection_range(0, END)
        return "break"


# (2) Widget Functions


def loan_amount(*args):
    num = zap_commas(loan_value.get())
    loan_value.set(num)
    ckval = number_check(loan_value.get(), 100, 1000000.01)
    if ckval is True:
        temp = round(float(num), 8)
        num = format(temp, " >#12.2f")
        loan_value.set(num)
        loan_entry.update()
        payment_entry.focus_set()
        payment_entry.selection_range(0, END)
    else:
        bad_entry_msg("loan amount", 'loan_entry', 'loan_value.set("100.01")')
        loan_entry.focus_set()
        return "break"


def pay_amount(*args):
    num = zap_commas(payment.get())
    payment.set(num)
    ckval = number_check(payment.get(), 1.00, float(loan_value.get()))
    if ckval is True:
        temp = round(float(num), 8)
        round(temp, 3)
        num = format(temp, " >#10.2f")
        payment.set(num)
        payment_entry.update()
        apr_entry.focus_set()  # on to the next data to enter
        apr_entry.selection_range(0, END)
    else:
        bad_entry_msg("payment amount", 'payment_entry', 'payment.set("10.01")')
        payment_entry.focus_set()
        return "break"


def apr_amount(*args):
    num = zap_commas(apr.get())
    apr.set(num)
    val_check = number_check(apr.get(), .1, 24)
    if val_check is True:
        num = round(float(apr.get()), 8)
        apr.set(str(num))
        month_cbox.focus_set()
    else:
        bad_entry_msg("APR amount", "apr_entry", "apr.set('3.5')")
        apr_entry.focus_set()
        return 'break'   # break interrupts the process and keeps focus on the apr entry


def show_results():
    if start_month.get() == "Month":
        start_month.set('Jan')
    if start_yr.get() == "Year":
        start_yr.set('1980')
# prepare to create header and table
    c1 = "Period"  # header tezt for each column
    c2 = "$ Interest"
    c3 = "$ Principal"
    c4 = "$ New Balance"
    c5 = "Payment Date"
    table_txt.delete(1.0, END)
    lnamt = float(loan_value.get())
    p = float(payment.get())
    apr_num = float(apr.get())/100
    apr2show = format(apr_num, '>5.3%')
    rpermo = apr_num/12
    mopr2show = format(rpermo, '<6.4%')
    # calculate the number of periods - if it is > 370 abort
    num_pay = 0
    try:
        num_pay = (-math.log10(1-(rpermo*lnamt/p)))/(math.log10(1+rpermo))
    except ValueError:
        mymsg = "Payment is insufficient to resolve loan."
        badnews(mymsg)
        exit()
    if num_pay > 361:
        mymsg = "Inconsistent data entered or loan is > 30 years: " + str(num_pay) + " periods."
        badnews(mymsg)
    spacer = "   "
    rtn = "\n"
    table_txt.insert(END, format("Loan Amount: ", '<30s') + format(lnamt, '>15,.2f') + rtn)
    table_txt.insert(END, format("Payment Frequency: ", '<30s') + format('Monthly', '>15s') + rtn)
    table_txt.insert(END, format("Monthly Payment: ", '<30s') + format(float(p), '>15,.2f') + rtn)
    table_txt.insert(END, format("Annual Interest Rate (ARP): ", '<30s') + format(apr2show, '>15s') + rtn)
    table_txt.insert(END, format("Monthly Compound Rate: ", '<30s') + format(mopr2show, '>15s') + rtn)
    table_txt.insert(END, format("Starting Month and Year: ", '<30s') + format((start_month.get() + " " + start_yr.get()), '>15s') + rtn)
    table_txt.insert(END, format("Number of Payments: ", '<30s') + format(num_pay, '>15,.2f') + rtn)
    mopaynum = divmod(num_pay, 1)
    fullpay = mopaynum[0]
    partpay = mopaynum[1]
    table_txt.insert(END, spacer + format(fullpay, '<3.0f') + " full monthly payments" + rtn)
    if partpay > 0:
        table_txt.insert(END, spacer + "plus 1 final payment of about " + format((partpay * float(p)), "<6.2f") + rtn*2)
# display header
    table_txt.insert(END, f"{c1: <8s}{c5: <12s}{spacer}{c2: <10s}{spacer}{c3: <12s}{spacer}{c4: <13s}{rtn}")
# set up table
    period = 1
    mo_payment = float(payment.get())
    month = start_month.get()
    year = start_yr.get()
    per_str = month + " " + year   # string with month and year
    year = int(year)
    principal = float(loan_value.get())
    new_balance = principal
    interest = principal * rpermo
    principal_reduced = mo_payment - interest
    new_balance -= principal_reduced
# create and display table
    while interest > 0:
        table_txt.insert(END, f"{str(period): <8s}{str(per_str): >12s}{spacer}{interest: > 10,.2f}{spacer}{principal_reduced: > 11,.2f}{spacer}{new_balance: > 13,.2f}{rtn}")
        period += 1
        indexno = months.index(month)
        indexno += 1
        if indexno == 12:
            indexno = 0
            year += 1
        month = months[indexno]
        per_str = month + " " + str(year)
        if new_balance <= mo_payment:
            interest = new_balance * rpermo
            principal_reduced = new_balance
            new_balance = 0
        else:
            interest = new_balance * rpermo
            principal_reduced = float(payment.get()) - interest
            new_balance -= principal_reduced


# ____________________________

# Section 3: Widget Objects
# Remember - creating everything inside top1, our work window


# starting out with a pair of labelframes, 1 for inputs, 1 for table display
data_frame = LabelFrame(top1, width=300, height=748, borderwidth=10)
table_frame = LabelFrame(top1, width=904, height=748, borderwidth=10)
data_frame.pack(side=LEFT, padx=5, pady=5, expand=FALSE, fill="y")
table_frame.pack(side=LEFT, padx=5, pady=5, expand=FALSE, fill="y")
# alternatively, we could use:
# data_frame.grid(row=1, column=0)
# table_frame.grid(row=1, column=1)
data_frame.pack_propagate(FALSE)
table_frame.pack_propagate(FALSE)


# ~~~
# Now fill the Data Frame with data related widgets
# ------ frame header stuff --------
# a title
title = Label(data_frame, text="Simple Amortization Table", justify=CENTER)
title.grid(row=1, column=1, sticky=NW)
title.configure(font="14")


# spacer
spacer1 = LabelFrame(data_frame)
spacer1.grid(row=2, column=1, pady=5)


# a notes/assumptions text box
notes = Text(data_frame,  height=4, width=40)
notes.grid(row=3, column=1, padx=15)
assumptions = "Assumptions:\nMonthly compounding with payments \ndue monthly at the end of the month.\n30 year + 1 mo max term"
notes.insert(END, assumptions)


# spacer
spacer2 = LabelFrame(data_frame)
spacer2.grid(row=4, column=1, pady=10)


# ------ this is where the data is collected--------

# loan amount label and entry box

lltxt = "Enter Loan Amount: $(100 -> 1000000)"
loan_frame = Frame(data_frame, height=3)
loan_label = Label(loan_frame, text=lltxt)
loan_entry = Entry(loan_frame,  textvariable=loan_value)
loan_frame.grid(row=5, column=1, sticky=W)
loan_label.grid(row=0, column=0, sticky=W)
loan_entry.grid(row=1, column=0, pady=10, sticky=W)
loan_entry.bind('<Return>', loan_amount)
loan_entry.bind('<Tab>', loan_amount)
loan_entry.bind('<1>', clickcheck)

# spacer
spacer3 = LabelFrame(data_frame)
spacer3.grid(row=6, column=1, pady=10)

# loan payment amount label and entry box
loan_pay_txt = "Enter Loan Payment Amount: $(1 -> loan amount)"
payment_frame = Frame(data_frame, height=3)
payment_label = Label(payment_frame, text=loan_pay_txt)
payment_entry = Entry(payment_frame,  textvariable=payment)
payment_frame.grid(row=7, column=1, sticky=W)
payment_label.grid(row=0, column=0, sticky=W)
payment_entry.grid(row=1, column=0, pady=10, sticky=W)
payment_entry.bind('<Return>', pay_amount)
payment_entry.bind('<Tab>', pay_amount)
payment_entry.bind('<1>', clickcheck)

# spacer
spacer4 = LabelFrame(data_frame)
spacer4.grid(row=8, column=1, pady=10)

# APR amount label and entry box
apr_label_txt = "Enter APR (annual percentage rate):\nFor example 7.5 for 7.5% annual rate.\n" \
                "( .1 -> 24 ) Note: 24 is the max legal in U.S. (TN)"
apr_frame = Frame(data_frame, height=2)
apr_label = Label(apr_frame, text=apr_label_txt)
apr_entry = Entry(apr_frame, textvariable=apr)
apr_frame.grid(row=9, column=1, sticky=W)
apr_label.grid(row=0, column=0, sticky=W)
apr_entry.grid(row=1, column=0, pady=10, sticky=W)
apr_entry.bind('<Return>', apr_amount)
apr_entry.bind('<Tab>', apr_amount)
apr_entry.bind('<1>', clickcheck)

# spacer
spacer5 = LabelFrame(data_frame)
spacer5.grid(row=10, column=1, pady=10)

# date input grid
date_frame = Frame(data_frame)
date_label = Label(date_frame, text=date_label_header)
date_frame.grid(row=11, column=1, sticky=W)
date_label.grid(row=0, column=0, sticky=W, columnspan=3)
month_cbox = Combobox(date_frame, values=months, width=12, textvariable=start_month, state="readonly")
month_cbox.grid(row=2, column=0, sticky=E+W)
year_cbox = Combobox(date_frame, values=years, width=12, textvariable=start_yr, state="readonly")
year_cbox.grid(row=2, column=1, sticky=E+W, padx=10)
month_cbox.bind('<1>', clickcheck)
year_cbox.bind('<1>', clickcheck)

# spacer
spacer6 = LabelFrame(data_frame)
spacer6.grid(row=13, column=1, pady=20)

# calculate button
b1 = Button(data_frame, text="LEFT CLICK TO CALCULATE AMORTIZATION TABLE", command=show_results, width=24)
b1.grid(row=15, column=1, columnspan=3, sticky="nsew")

# In the table frame next to the data frame create text display to hold the loan table

table_txt = Text(table_frame, width=600)
table_txt.pack(fill="both", expand=TRUE)
sbar1 = Scrollbar(table_txt, orient="vertical", command=table_txt.yview)
sbar1.pack(side="right", anchor="ne", fill="y")
table_txt.configure(yscrollcommand=sbar1.set(0, .1))

# ____________________________
# Section4: Startup Code

setup = TRUE
loan_entry.focus_set()
loan_entry.selection_range(0, END)
top1.update()

# starts execution
root.mainloop()