Added what-if payments.

Hypothetical extra payments towards principal without
modifying files.

Makefile

@@ -3,5 +3,5 @@ reference:
 	amortize -P 100000 -r 0.05 -f monthly -n 36 -s > ref.txt
 
 test:
-	python amort.py -p 100000 -i 5.0 -t 3 -ot "{\"payment_number\":13,\"amount\":5000}"
+	python src/amort.py -p 100000 -i 5.0 -t 3 -ot "{\"payment-number\":13,\"amount\":5000}"
 

README.md

@@ -0,0 +1,45 @@
+
+# amort
+
+Generates an amortization schedule based on the principal, interest rate, term, and any additional payments towards principal.
+
+## requirements
+
+Only imports `argparse` and `json` for the main block of the script.
+
+* saves results to a file `schedule.csv`
+* does not access the web or internet
+
+## usage
+
+Invoke `make test` to generate a schedule based on loan of 100,000 over a 3 year term given an interest rate of 5.0% with a one-time payment towards principal of 5000 at the 13th month.
+
+### extra payments
+
+For extra payments towards principal, invoking the script will look like:
+
+    python src/amort.py -p {principal} -i {interest rate} -t {years} --extra-payments extra_payments.json
+
+A valid json file contains a list named `extra-payments`. Each element is an object with the attributes `payment-number` and `amount`.
+
+    {
+        "extra-payments" : [
+            {
+                "payment-number": 1,
+                "amount": 500
+            }, {
+                "payment-number": 2,
+                "amount": 500
+            }, {
+                "payment-number": 3,
+                "amount": 500
+            }
+        ]
+    }
+
+* `payment-number` is the month when the payment has been made.
+* `amount` is self-explanatory
+
+## misc
+
+This originated from me playing with ChatGPT. I asked it a question and it blurted out a semi-functional script as an answer. Then I made this repo to track the errors I fixed.

amort.py

@@ -1,132 +0,0 @@
-
-import argparse, json
-
-def generate_amortization_schedule(principal, interest_rate, loan_term, extra_payments=[]):
-    # Convert interest rate to decimal and calculate periodic interest rate
-    monthly_interest_rate = interest_rate / 12 / 100
-
-    # Calculate the number of monthly payments
-    num_payments = loan_term * 12
-
-    # Calculate the fixed monthly payment using the amortization formula
-    monthly_payment = (principal * monthly_interest_rate) / (1 - (1 + monthly_interest_rate) ** -num_payments)
-    monthly_payment = round(monthly_payment, 2)
-
-    # Initialize variables
-    remaining_balance = principal
-    amortization_schedule = []
-
-    one_time_payment = None
-    if extra_payments != []:
-        one_time_payment = extra_payments.pop(0)
-    for payment_number in range(1, num_payments + 1):
-        # Calculate interest for the current period
-        if remaining_balance == 0:
-            break
-        interest_payment = round(remaining_balance * monthly_interest_rate, 2)
-
-        # Calculate principal payment
-        principal_payment = round(monthly_payment - interest_payment, 2)
-
-        # Apply one-time payment if provided
-        if one_time_payment and payment_number == one_time_payment['payment_number']:
-            principal_payment += one_time_payment['amount']
-            if extra_payments != []:
-                one_time_payment = extra_payments.pop(0)
-        # Update remaining balance
-        remaining_balance -= principal_payment
-
-        if remaining_balance < 0:
-            principal_payment = principal_payment + remaining_balance
-            monthly_payment = monthly_payment + remaining_balance
-            remaining_balance = 0
-
-        # Create a dictionary with payment details and add it to the amortization schedule
-        payment_details = {
-            'Payment Number': payment_number,
-            'Payment Amount': round(monthly_payment,2),
-            'Interest Payment': interest_payment,
-            'Principal Payment': round(principal_payment,2),
-            'Remaining Balance': round(remaining_balance,2)
-        }
-        amortization_schedule.append(payment_details)
-    return amortization_schedule
-
-def get_totals(amortization_schedule, func=None):
-    total_paid = 0
-    total_interest_paid = 0
-    total_principal_paid = 0
-    # Display the amortization schedule
-    messages = []
-    for payment in amortization_schedule:
-        # print(payment)
-        total_paid += payment["Payment Amount"]
-        total_interest_paid += payment["Interest Payment"]
-        total_principal_paid += payment["Principal Payment"]  
-        if payment["Remaining Balance"] < 0:
-            break
-        attrs = [payment[key] for key in payment]
-        if func is not None:
-            messages.append("%s" % ", ".join([str(attr) for attr in attrs]))
-    if func is not None:
-        func(messages)
-    return total_paid, total_interest_paid, total_principal_paid
-
-if __name__ == "__main__":
-    def get_arguments():
-        p = argparse.ArgumentParser()
-        p. add_argument("--principal", "-p", type=float, \
-            help="set value for principal")
-        p.add_argument("---interest-rate", "-i", type=float,\
-            help="set the value for interest rate (percentage)")
-        p.add_argument("--term", "-t", type=int,\
-            help="sets the term (years)")
-        p.add_argument("--one-time", "-ot", type=str,\
-            help="factors in a one-time payment (json, example: {\"payment_number\":13,\"amount\":5000}")
-        p.add_argument("--extra-payments", "-ep", type=str,\
-            help="facts in multiple one time payments (json file name)")
-        args = p.parse_args()
-        l = []
-        if args.extra_payments is not None:
-            with open(args.extra_payments) as f:
-                l = json.loads(f.read())
-                # print(extra["extra-payments"])
-        if args.one_time is not None:
-            l.append(json.loads(args.one_time))
-        extra = l["extra-payments"]
-        extra.sort(key=lambda k: k["payment_number"])
-        return args.principal, args.interest_rate, args.term, extra
-
-    def compare(with_extra_payments, without):
-        x,y,z = get_totals(with_extra_payments)
-        a,b,c = get_totals(without)
-        print(chr(916), "paid: ", round(x-a,2))
-        print(chr(916), "interest paid: ", round(y-b,2))
-        print(chr(916), "principal paid: ", round(abs(z-c),2))
-
-    def display(table):
-        print("id, paid, interest payment, principal payment, remaining")
-        for row in table:
-            print(row)
-
-    def export(table, filename="schedule.csv"):
-        with open(filename, 'w') as f:
-            print("id, paid, interest payment, principal payment, remaining", file=f)
-            for row in table:
-                print(row, file=f)
-        print("wrote to file", filename)
-
-    principal, interest_rate, loan_term, extra_payments = get_arguments()
-
-    schedule = generate_amortization_schedule(principal, interest_rate, loan_term, extra_payments)
-    paid, interest_paid, principal_paid = get_totals(schedule,export)
-
-    print("total paid: ", round(paid,2))
-    print("total interest paid: ", round(interest_paid,2))
-    print("total principal paid: ", round(principal_paid,2))
-
-    # without extra payments for comparison
-    compare(schedule, generate_amortization_schedule(
-        principal, interest_rate, loan_term)
-    )
-

src/amort.py

@@ -0,0 +1,183 @@
+
+def generate_amortization_schedule(principal, interest_rate, loan_term, events=[]):
+    # Convert interest rate to decimal and calculate periodic interest rate
+    monthly_interest_rate = interest_rate / 12 / 100
+
+    # Calculate the number of monthly payments
+    num_payments = loan_term * 12
+
+    # Calculate the fixed monthly payment using the amortization formula
+    monthly_payment = (principal * monthly_interest_rate) / (1 - (1 + monthly_interest_rate) ** -num_payments)
+    monthly_payment = round(monthly_payment, 2)
+
+    # Initialize variables
+    remaining_balance = principal
+    amortization_schedule = []
+
+    one_time_payment = None
+    if events != []:
+        one_time_payment = events["extra-payments"].pop(0)
+
+    for payment_number in range(1, num_payments + 1):
+        # Calculate interest for the current period
+        if remaining_balance == 0:
+            break
+        interest_payment = round(remaining_balance * monthly_interest_rate, 2)
+
+        # Calculate principal payment
+        principal_payment = round(monthly_payment - interest_payment, 2)
+
+        # Apply one-time payment if provided
+        if one_time_payment and payment_number == one_time_payment['payment-number']:
+            principal_payment += one_time_payment['amount']
+            if events["extra-payments"] != []:
+                one_time_payment = events["extra-payments"].pop(0)
+        # Update remaining balance
+        remaining_balance -= principal_payment
+
+        if remaining_balance < 0:
+            principal_payment = principal_payment + remaining_balance
+            monthly_payment = monthly_payment + remaining_balance
+            remaining_balance = 0
+
+        # Create a dictionary with payment details and add it to the amortization schedule
+        payment_details = {
+            'Payment Number': payment_number,
+            'Payment Amount': round(monthly_payment,2),
+            'Interest Payment': interest_payment,
+            'Principal Payment': round(principal_payment,2),
+            'Remaining Balance': round(remaining_balance,2)
+        }
+        amortization_schedule.append(payment_details)
+    return amortization_schedule
+
+def get_totals(amortization_schedule, func=None):
+    total_paid = 0
+    total_interest_paid = 0
+    total_principal_paid = 0
+    # Display the amortization schedule
+    messages = []
+    for payment in amortization_schedule:
+        # total_paid += payment["Payment Amount"]
+        total_interest_paid += payment["Interest Payment"]
+        total_principal_paid += payment["Principal Payment"]  
+        if payment["Remaining Balance"] < 0:
+            break
+        attrs = [payment[key] for key in payment]
+        if func is not None:
+            messages.append("%s" % ", ".join([str(attr) for attr in attrs]))
+    if func is not None:
+        func(messages)
+    return ((total_interest_paid + total_principal_paid), 
+            total_interest_paid, 
+            total_principal_paid)
+
+def compare(with_extra_payments, without):
+    x,y,z = get_totals(with_extra_payments)
+    a,b,c = get_totals(without)
+    term_length = len(without) - len(with_extra_payments)
+    print(chr(916), "paid: ", round(x-a,2))
+    print(chr(916), "interest paid: ", round(y-b,2))
+    print(chr(916), "principal paid: ", round(abs(z-c),2))
+    print(chr(916), "term (months): ", term_length)
+
+def whatif(events, x = 500, terms = 120):
+    """
+    What if I paid an extra x dollars each term.
+    Parameters:
+        x     = amount to pay
+        terms = total expected terms
+    """
+    key_list = "extra-payments"
+    key_attr1 = "payment-number"
+    key_attr2 = "amount"
+    # get the last payment number from events
+    latest = events[key_list][-1][key_attr1]
+    start  = latest + 1
+    for i in range(start, 120):
+        new_event = dict()
+        new_event[key_attr1] = i
+        new_event[key_attr2] = x
+        events[key_list].append(new_event)
+    return events, start
+
+def display(table):
+    print("id, due, interest payment, principal payment, remaining")
+    for row in table:
+        print(row)
+
+def export(table, filename="schedule.csv"):
+    with open(filename, 'w') as f:
+        print("id, due, interest payment, principal payment, remaining", file=f)
+        for row in table:
+            print(row, file=f)
+    print("wrote to file", filename)
+
+if __name__ == "__main__":
+    import argparse, json, copy
+
+    def get_arguments():
+        p = argparse.ArgumentParser()
+        p. add_argument("--principal", "-p", type=float, \
+            help="set value for principal")
+        p.add_argument("---interest-rate", "-i", type=float,\
+            help="set the value for interest rate (percentage)")
+        p.add_argument("--term", "-t", type=int,\
+            help="sets the term (years)")
+        p.add_argument("--one-time", "-ot", type=str,\
+            help="a one-time payment (json, example: {\"payment-number\":13,\"amount\":5000}")
+        p.add_argument("--events", "-e", type=str,\
+            help="name of .json file containing events such as one-time payments or interest rate changes")
+        args = p.parse_args()
+        l = []
+        if args.events is not None:
+            with open(args.events) as f:
+                l = json.loads(f.read())
+        if args.one_time is not None:
+           l["extra-payments"].append(json.loads(args.one_time))
+        return args.principal, args.interest_rate, args.term, l
+
+    def main():
+        principal, interest_rate, loan_term, events = get_arguments()
+
+        wi_start = 0
+        wi_x     = 500
+        if events:
+            # the amount paid ahead canonically
+            ahead_paid = sum(pay["amount"] for pay in events["extra-payments"])
+            ahead_msg  = f"total extra payments: {ahead_paid}"
+            border = "~" * len(ahead_msg)
+            print(border)
+            print(ahead_msg)
+            print(border)
+            events, wi_start = whatif(events, x=wi_x)
+        # generate the schedule
+        schedule = generate_amortization_schedule(
+            principal, interest_rate, loan_term, copy.deepcopy(events)
+        )
+        paid, interest_paid, principal_paid = get_totals(schedule,export)
+        # report on schedule
+        print("total paid: ", round(paid,2))
+        print("total interest paid: ", round(interest_paid,2))
+        print("total principal paid: ", round(principal_paid,2))
+        # get the last term of the schedule
+        if events:
+            wi_end = schedule[-1]["Payment Number"]
+            wi_paid = sum(
+                pay["amount"] for pay in \
+                    events["extra-payments"][wi_start:wi_end]
+            )
+            wi_msg = f"total if {wi_x} was also paid each term: {wi_paid}"
+            border = '~' * len(wi_msg)
+            print(border)
+            print(wi_msg)
+            print(border)
+            # without extra payments for comparison
+            compare(schedule, generate_amortization_schedule(
+                principal, interest_rate, loan_term)
+            )
+        print()
+    # with open("extra_payments.json") as f:
+    #     e = json.load(f)
+    # print(whatif(e))
+    main()