data: Add PostingMeta.detached() method.

conservancy_beancount/data.py

@@ -512,10 +512,9 @@ class PostingMeta(Metadata):
         self.txn = txn
         self.index = index
         self.post = post
-        if post.meta is None:
-            self.meta = self.txn.meta
-        else:
-            self.meta = collections.ChainMap(post.meta, txn.meta)
+        self.meta: collections.ChainMap = collections.ChainMap(txn.meta)
+        if post.meta is not None:
+            self.meta = self.meta.new_child(post.meta)
 
     def __getitem__(self, key: MetaKey) -> MetaValue:
         try:
@@ -527,17 +526,16 @@ class PostingMeta(Metadata):
                 raise
 
     def __setitem__(self, key: MetaKey, value: MetaValue) -> None:
-        if self.post.meta is None:
+        if len(self.meta.maps) == 1:
             self.post = self.post._replace(meta={key: value})
+            assert self.post.meta is not None
             self.txn.postings[self.index] = self.post
-            # mypy complains that self.post.meta could be None, but we know
-            # from two lines up that it's not.
-            self.meta = collections.ChainMap(self.post.meta, self.txn.meta)  # type:ignore[arg-type]
+            self.meta = self.meta.new_child(self.post.meta)
         else:
             super().__setitem__(key, value)
 
     def __delitem__(self, key: MetaKey) -> None:
-        if self.post.meta is None:
+        if len(self.meta.maps) == 1:
             raise KeyError(key)
         else:
             super().__delitem__(key)
@@ -550,6 +548,17 @@ class PostingMeta(Metadata):
     def date(self) -> datetime.date:
         return self.txn.date
 
+    def detached(self) -> 'PostingMeta':
+        """Create a copy of this PostingMeta detached from the original post
+
+        Changes you make to the detached copy will not propagate to the
+        underlying data structures. This is mostly useful for reporting code
+        that may want to "split" and manipulate the metadata multiple times.
+        """
+        retval = type(self)(self.txn, self.index, self.post)
+        retval.meta = retval.meta.new_child()
+        return retval
+
 
 class Posting(BasePosting):
     """Enhanced Posting objects

tests/test_data_posting_meta.py

@@ -126,6 +126,24 @@ def test_date(date):
     for index, post in enumerate(txn.postings):
         assert data.PostingMeta(txn, index, post).date == date
 
+def test_mutable_copy():
+    txn = testutil.Transaction(
+        filename='f', lineno=130, txnkey='one', postings=[
+        ('Assets:Cash', 18),
+        ('Income:Donations', -18),
+    ])
+    meta = data.PostingMeta(txn, 1).detached()
+    meta['layerkey'] = 'two'
+    assert dict(meta) == {
+        'filename': 'f',
+        'lineno': 130,
+        'txnkey': 'one',
+        'layerkey': 'two',
+    }
+    assert 'layerkey' not in txn.meta
+    assert all(post.meta is None for post in txn.postings)
+    assert meta.date == txn.date
+
 # The .get() tests are arguably testing the stdlib, but they're short and
 # they confirm that we're using the stdlib as we intend.
 def test_get_with_meta_value(simple_txn):