reports: Separate methods for Balances to get posting classification.

For easier subclass customization.

conservancy_beancount/reports/core.py

@@ -353,18 +353,24 @@ class Balances:
                 fund = Fund.UNRESTRICTED
             else:
                 fund = Fund.RESTRICTED
-            try:
+            classification = self._get_classification(post)
-                classification_s = post.account.meta['classification']
-                if isinstance(classification_s, str):
-                    classification = data.Account(classification_s)
-                else:
-                    raise TypeError()
-            except (KeyError, TypeError):
-                classification = post.account
             post_meta = post.meta.get(post_meta_key)
             key = BalanceKey(post.account, classification, period, fund, post_meta)
             self.balances[key] += post.at_cost()
 
+    def _get_classification(self, post: data.Posting) -> data.Account:
+        try:
+            return self._get_meta_account(post.account.meta, 'classification')
+        except (KeyError, TypeError):
+            return post.account
+
+    def _get_meta_account(self, meta: Mapping[MetaKey, MetaValue], key: MetaKey) -> data.Account:
+        value = meta[key]
+        if isinstance(value, str):
+            return data.Account(value)
+        else:
+            raise TypeError(f"{key!r} is not a string but a {type(value).__name__}")
+
     def total(self,
               account: Union[None, str, Collection[str]]=None,
               classification: Optional[str]=None,