config: Add Config.books_loader() method.

conservancy_beancount/config.py

@@ -111,6 +111,13 @@ class Config:
             retval = default or (Path.home() / self._ENVIRON_DEFAULT_PATHS[key])
         return retval
 
+    def books_loader(self) -> Optional[books.Loader]:
+        books_path = self.books_path()
+        if books_path is None:
+            return None
+        else:
+            return books.Loader(books_path, self.fiscal_year_begin())
+
     def books_path(self) -> Optional[Path]:
         try:
             retval = Path(self.file_config['Beancount'].get('books dir'))

tests/test_config.py

@@ -381,3 +381,14 @@ def test_default_fiscal_year_begin():
     actual = config.fiscal_year_begin()
     assert actual.month == 3
     assert actual.day == 1
+
+def test_books_loader():
+    books_path = testutil.test_path('bookstest')
+    config = config_mod.Config()
+    config.load_string(f'[Beancount]\nbooks dir = {books_path}\n')
+    loader = config.books_loader()
+    expected = 'include "{}"'.format(books_path / 'books/2020.beancount')
+    assert loader.fy_range_string(0, 2020, {}) == expected
+
+def test_books_loader_without_books():
+    assert config_mod.Config().books_loader() is None