data: Fix Amount.__new__.

See the comments for background and rationale.

conservancy_beancount/data.py

@@ -132,6 +132,15 @@ class Amount(bc_amount.Amount):
     """
     number: decimal.Decimal
 
+    # beancount.core._Amount is the plain namedtuple.
+    # beancore.core.Amount adds instance methods to it.
+    # b.c.Amount.__New__ calls `b.c._Amount.__new__`, which confuses type
+    # checking. See <https://github.com/python/mypy/issues/1279>.
+    # It works fine if you use super(), which is better practice anyway.
+    # So we override __new__ just to call _Amount.__new__ this way.
+    def __new__(cls, number: decimal.Decimal, currency: str) -> 'Amount':
+        return super(bc_amount._Amount, Amount).__new__(cls, number, currency)
+
 
 class Metadata(MutableMapping[MetaKey, MetaValue]):
     """Transaction or posting metadata
@@ -309,7 +318,7 @@ def balance_of(txn: Transaction,
         ]
         number = sum((wt.number for wt in weights), number)
         currency = weights[0].currency
-    return Amount._make((number, currency))
+    return Amount(number, currency)
 
 @functools.lru_cache()
 def is_opening_balance_txn(txn: Transaction) -> bool:

tests/testutil.py

@@ -74,12 +74,12 @@ def Cost(number, currency='USD', date=FY_MID_DATE, label=None):
 
 def Posting(account, number,
             currency='USD', cost=None, price=None, flag=None,
-            **meta):
+            type_=bc_data.Posting, **meta):
     if cost is not None:
         cost = Cost(*cost)
-    if meta is None:
+    if not meta:
         meta = None
-    return bc_data.Posting(
+    return type_(
         account,
         Amount(number, currency),
         cost,
@@ -132,6 +132,7 @@ class Transaction:
             'lineno': 0,
         }
         self.meta.update(meta)
+        if postings is not None:
             for posting in postings:
                 self.add_posting(*posting)