Added what-if payments.

Hypothetical extra payments towards principal without
modifying files.

src/amort.py

@@ -81,6 +81,26 @@ 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, due, interest payment, principal payment, remaining")
     for row in table:
@@ -94,7 +114,7 @@ def export(table, filename="schedule.csv"):
     print("wrote to file", filename)
 
 if __name__ == "__main__":
-    import argparse, json
+    import argparse, json, copy
 
     def get_arguments():
         p = argparse.ArgumentParser()
@@ -117,29 +137,47 @@ if __name__ == "__main__":
            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()
 
-    principal, interest_rate, loan_term, events = get_arguments()
-
-    if events != []:
-        ahead_paid = float()
-        for extra_payment in events["extra-payments"]:
-            ahead_paid += extra_payment["amount"]
-        print("~~~~~~~~~~~~~~~~~~~~~")
-        print("total ahead payments:", ahead_paid)
-        print("~~~~~~~~~~~~~~~~~~~~~")
-
-    schedule = generate_amortization_schedule(
-        principal, interest_rate, loan_term, events
-    )
-    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)
-    )
-    print()
-
+        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()