cdjk · January 1, 2023 12:03 · Sep 6, 2017 · Sep 6, 2017 · Sep 6, 2017
diff --git a/example.beancount b/example.beancount
@@ -0,0 +1,15 @@
+plugin "amortize_over"
+
+2017-06-01 open Assets:Bank:Checking
+2017-06-01 open Assets:Prepaid-Expenses
+2017-06-01 open Expenses:Insurance:Auto
+
+2017-06-01 * "Pay car insurance"
+    Assets:Bank:Checking       -600.00 USD
+    Assets:Prepaid-Expenses
+
+2017-06-01 * "Amortize car insurance over six months"
+    amortize_months: 6
+    Assets:Prepaid-Expenses       -600.00 USD
+    Expenses:Insurance:Auto
+
diff --git a/amortize_over.py b/amortize_over.py
@@ -1,3 +1,23 @@
+# Copyright (c) 2017 Cary Kempston
+
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
 import math
 
 from beancount.core.data import Transaction
@@ -9,14 +29,52 @@
 __plugins__ = ('amortize_over',)
 
 
+
+
 def amortize_over(entries, unused_options_map):
     """Repeat a transaction based on metadata.
 
     Args:
-      entries: A list of directives. We're interested only in the Transaction instances.
+      entries: A list of directives. We're interested only in the
+               Transaction instances.
       unused_options_map: A parser options dict.
     Returns:
       A list of entries and a list of errors.
+
+    Example use:
+
+    This plugin will convert the following transactions
+
+        2017-06-01 * "Pay car insurance"
+            Assets:Bank:Checking               -600.00 USD
+            Assets:Prepaid-Expenses
+
+        2017-06-01 * "Amortize car insurance over six months"
+            amortize_months: 3
+            Assets:Prepaid-Expenses            -600.00 USD
+            Expenses:Insurance:Auto
+
+    into the following transactions over six months:
+
+        2017/06/01 * Pay car insurance
+            Assets:Bank:Checking               -600.00 USD
+            Assets:Prepaid-Expenses             600.00 USD
+
+        2017/06/01 * Amortize car insurance over six months
+            Assets:Prepaid-Expenses            -200.00 USD
+            Expenses:Insurance:Auto             200.00 USD
+
+        2017/07/01 * Amortize car insurance over six months
+            Assets:Prepaid-Expenses            -200.00 USD
+            Expenses:Insurance:Auto             200.00 USD
+
+        2017/08/01 * Amortize car insurance over six months
+            Assets:Prepaid-Expenses            -200.00 USD
+            Expenses:Insurance:Auto             200.00 USD
+
+    Note that transactions are not included past today's date.  For example,
+    if the above transactions are processed on a date of 2017/07/25, the
+    transaction dated 2017/08/01 is not included.
     """
     new_entries = []
     errors = []
@@ -68,4 +126,4 @@ def amortize_transaction(entry):
         new_entry = new_entry._replace(date=entry.date + relativedelta(months=n_month))
         if new_entry.date <= date.today():
             new_entries.append(new_entry)
-    return new_entries
+    return new_entries
diff --git a/amortize_over.py b/amortize_over.py
@@ -0,0 +1,71 @@
+import math
+
+from beancount.core.data import Transaction
+from beancount.core.amount import Amount
+
+from datetime import date
+from dateutils import relativedelta
+
+__plugins__ = ('amortize_over',)
+
+
+def amortize_over(entries, unused_options_map):
+    """Repeat a transaction based on metadata.
+
+    Args:
+      entries: A list of directives. We're interested only in the Transaction instances.
+      unused_options_map: A parser options dict.
+    Returns:
+      A list of entries and a list of errors.
+    """
+    new_entries = []
+    errors = []
+
+    for entry in entries:
+        if isinstance(entry, Transaction) and 'amortize_months' in entry.meta:
+            new_entries.extend(amortize_transaction(entry))
+        else:
+            # Always replicate the existing entries - unless 'amortize_months'
+            # is in the metadata
+            new_entries.append(entry)
+
+    return new_entries, errors
+
+def split_amount(amount, periods):
+    if periods == 1:
+        return [ amount ]
+    amount_this_period = amount/periods
+    amount_this_period = amount_this_period.quantize(amount)
+    return [ amount_this_period ] + split_amount(amount-amount_this_period, periods-1)
+
+def amortize_transaction(entry):
+
+    if len(entry.postings) != 2:
+        raise ValueError('Amortized transactions must have exactly two postings.')
+
+    new_entries = []
+
+    original_postings = entry.postings
+
+    periods = entry.meta['amortize_months']
+
+    amount = abs(entry.postings[0].units.number)
+    currency = entry.postings[0].units.currency
+
+    monthly_amounts = split_amount(amount, periods)
+
+    for (n_month, monthly_number) in enumerate(monthly_amounts):
+        new_postings = []
+        for posting in entry.postings:
+            new_monthly_number = monthly_number
+            if posting.units.number < 0:
+                new_monthly_number = -monthly_number
+            new_posting = posting._replace(units=Amount(number=new_monthly_number,
+                                                        currency=currency))
+            new_postings.append(new_posting)
+
+        new_entry = entry._replace(postings=new_postings)
+        new_entry = new_entry._replace(date=entry.date + relativedelta(months=n_month))
+        if new_entry.date <= date.today():
+            new_entries.append(new_entry)
+    return new_entries
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