config: Nicer determination of default currency for converting/signing.

base defaults to USD more as an API restriction than anything else, so avoid
using it as a default setting.  Instead, use the user's books denomination
or locale setting.

oxrlib/config.py

@@ -5,6 +5,9 @@ import decimal
 import os.path
 import pathlib
 
+import babel
+import babel.numbers
+
 from . import cache, loaders
 
 HOME_PATH = pathlib.Path(os.path.expanduser('~'))
@@ -20,7 +23,8 @@ def currency_list(s):
 class Configuration:
     DATE_SEPS = frozenset('.-/ ')
     DEFAULT_CONFIG_PATH = pathlib.Path(HOME_PATH, '.config', 'oxrlib.ini')
-    NO_DENOMINATION = object()
+    LOCALE = babel.core.Locale.default()
+    SENTINEL = object()
     PREPOSITIONS = frozenset(['in', 'to', 'into'])
     TODAY = datetime.date.today()
 
@@ -106,7 +110,7 @@ class Configuration:
         )
         hist_parser.add_argument(
             '--no-denomination',
-            dest='denomination', action='store_const', const=self.NO_DENOMINATION,
+            dest='denomination', action='store_const', const=self.SENTINEL,
             help="Turn off an earlier --denomination setting",
         )
         hist_parser.add_argument(
@@ -134,7 +138,7 @@ class Configuration:
         hist_parser.add_argument(
             'word3', nargs='?', metavar='second code',
             help="Convert or show rates to this currency, in three-letter code format. "
-            "If not specified, defaults to the base currency.",
+            "If not specified, defaults to the user's preferred currency.",
         )
         hist_parser.add_argument('word4', nargs='?', help=argparse.SUPPRESS)
 
@@ -170,20 +174,27 @@ class Configuration:
             errmsg.append(repr(s_value))
             self.error(': '.join(errmsg))
 
+    def _user_currency(self, default=SENTINEL):
+        try:
+            return babel.numbers.get_territory_currencies(
+                self.LOCALE.territory, start_date=self.TODAY)[0]
+        except IndexError:
+            return default
+
     def _post_hook_historical(self):
         year = self.args.date.year
         if year < 100:
             # Don't let the user specify ambiguous dates.
             self.error("historical data not available from year {}".format(year))
         self._read_from_conffile('base', 'Historical', 'USD', currency_code)
-        if self.args.denomination is self.NO_DENOMINATION:
+        if self.args.denomination is self.SENTINEL:
             self.args.denomination = None
         else:
             self._read_from_conffile('denomination', 'Historical', None, currency_code)
-        self._read_from_conffile('signed_currencies', 'Historical', self.args.base,
+        pref_currency = self.args.denomination or self._user_currency(self.args.base)
+        self._read_from_conffile('signed_currencies', 'Historical', pref_currency,
                                  currency_list, convert_fallback=True)
         self._read_from_conffile('ledger', 'Historical', False, getter='getboolean')
-        self.args.to_currency = self.args.base
         raw_words = iter(getattr(self.args, 'word' + c) for c in '1234')
         words = iter(word for word in raw_words if word is not None)
         try:
@@ -202,7 +213,7 @@ class Configuration:
                 next_word = next(words, next_word)
             self.args.to_currency = self._convert_or_error(currency_code, next_word)
         except StopIteration:
-            pass
+            self.args.to_currency = pref_currency
 
     def _build_cache_loader(self):
         kwargs = dict(self.conffile.items('Cache'))

oxrlib_example.ini

@@ -35,5 +35,9 @@ ledger = no
 denomination = USD
 
 # Use signs for these currencies in Ledger output.
-# If not specified, defaults to the base currency.
+# If not specified, defaults to the user's preferred currency, which is
+# the first setting found from:
+# 1. the denomination setting above
+# 2. the user's locale
+# 3. the base currency (which defaults to USD)
 signed_currencies = USD, EUR

tests/test_Configuration.py

@@ -2,6 +2,7 @@ import datetime
 import decimal
 import os
 
+import babel
 import pytest
 
 from . import any_date, relpath
@@ -56,14 +57,17 @@ def test_historical_default_base(ini_filename, expected_currency, use_switch, an
     (decimal.Decimal('12.34'), 'gbp', 'IN', 'eur'),
 ])
 def test_historical_argparsing_success(amount, from_curr, preposition, to_curr, any_date):
+    oxrlib.config.Configuration.TODAY = datetime.date(2017, 1, 1)
+    # This locale's currency should not be used in any test cases above.
+    oxrlib.config.Configuration.LOCALE = babel.core.Locale('en', 'IN')
     arglist = ['historical', any_date.isoformat()]
     arglist.extend(str(s) for s in [amount, from_curr, preposition, to_curr]
                    if s is not None)
     config = config_from(os.devnull, arglist)
+    expect_to_curr = 'INR' if to_curr is None else to_curr.upper()
     assert config.args.amount == amount
     assert config.args.from_currency == from_curr.upper()
-    if to_curr is not None:
-        assert config.args.to_currency == to_curr.upper()
+    assert config.args.to_currency == expect_to_curr
 
 @pytest.mark.parametrize('arglist', [
     ['100'],