Added what-if payments.

Hypothetical extra payments towards principal without
modifying files.

src/amort.py

@@ -1,5 +1,5 @@
 
-def generate_amortization_schedule(principal, interest_rate, loan_term, extra_payments=[]):
+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
 
@@ -15,8 +15,9 @@ def generate_amortization_schedule(principal, interest_rate, loan_term, extra_pa
     amortization_schedule = []
 
     one_time_payment = None
-    if extra_payments != []:
+    if events != []:
-        one_time_payment = extra_payments.pop(0)
+        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:
@@ -29,8 +30,8 @@ def generate_amortization_schedule(principal, interest_rate, loan_term, extra_pa
         # 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 != []:
+            if events["extra-payments"] != []:
-                one_time_payment = extra_payments.pop(0)
+                one_time_payment = events["extra-payments"].pop(0)
         # Update remaining balance
         remaining_balance -= principal_payment
 
@@ -57,8 +58,7 @@ def get_totals(amortization_schedule, func=None):
     # Display the amortization schedule
     messages = []
     for payment in amortization_schedule:
-        # print(payment)
+        # total_paid += payment["Payment Amount"]
-        total_paid += payment["Payment Amount"]
         total_interest_paid += payment["Interest Payment"]
         total_principal_paid += payment["Principal Payment"]  
         if payment["Remaining Balance"] < 0:
@@ -68,7 +68,9 @@ def get_totals(amortization_schedule, func=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
+    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)
@@ -79,20 +81,40 @@ def compare(with_extra_payments, without):
     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, paid, interest payment, principal payment, remaining")
+    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, paid, interest payment, principal payment, remaining", file=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
+    import argparse, json, copy
 
     def get_arguments():
         p = argparse.ArgumentParser()
@@ -103,33 +125,59 @@ if __name__ == "__main__":
         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}")
+            help="a one-time payment (json, example: {\"payment-number\":13,\"amount\":5000}")
-        p.add_argument("--extra-payments", "-ep", type=str,\
+        p.add_argument("--events", "-e", type=str,\
-            help="facts in multiple one time payments (json file name)")
+            help="name of .json file containing events such as one-time payments or interest rate changes")
         args = p.parse_args()
         l = []
-        if args.extra_payments is not None:
+        if args.events is not None:
-            with open(args.extra_payments) as f:
+            with open(args.events) as f:
                 l = json.loads(f.read())
-        extra = []
-        if "extra-payments" in l:
-            extra = l["extra-payments"]
-            extra.sort(key=lambda k: k["payment-number"])
         if args.one_time is not None:
-            extra.append(json.loads(args.one_time))
+           l["extra-payments"].append(json.loads(args.one_time))
-        return args.principal, args.interest_rate, args.term, extra
+        return args.principal, args.interest_rate, args.term, l
 
-    principal, interest_rate, loan_term, extra_payments = get_arguments()
+    def main():
-
+        principal, interest_rate, loan_term, events = 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)
-    )
 
+        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()