reports: Balance.format() accepts None as a format.

conservancy_beancount/reports/core.py

@@ -113,7 +113,7 @@ class Balance(Mapping[str, data.Amount]):
         return self._all_amounts(operator.le, 0)
 
     def format(self,
-               fmt: str='#,#00.00 ¤¤',
+               fmt: Optional[str]='#,#00.00 ¤¤',
                sep: str=', ',
                empty: str="Zero balance",
     ) -> str:
@@ -122,6 +122,9 @@ class Balance(Mapping[str, data.Amount]):
         If the balance is zero, returns ``empty``. Otherwise, returns a string
         with each amount in the balance formatted as ``fmt``, separated by
         ``sep``.
+
+        If you set ``fmt`` to None, amounts will be formatted according to the
+        user's locale. The default format is Beancount's input format.
         """
         amounts = [amount for amount in self.values() if amount.number]
         if not amounts:

tests/test_reports_balance.py

@@ -22,6 +22,8 @@ import pytest
 
 from . import testutil
 
+import babel.numbers
+
 from conservancy_beancount.reports import core
 
 DEFAULT_STRINGS = [
@@ -191,6 +193,12 @@ def test_format_sep(sep):
     balance = core.Balance(amounts)
     assert balance.format(sep=sep) == expected
 
+def test_format_none():
+    amounts = testutil.balance_map(BRL=65000)
+    balance = core.Balance(amounts)
+    expected = babel.numbers.format_currency(65000, 'BRL')
+    assert balance.format(None) == expected
+
 @pytest.mark.parametrize('empty', [
     "N/A",
     "Zero",