data: Move iter_posting into Posting class methods.

As I move into reporting code, having Posting.from_beancount() is
handy, and then from_txn() might as well come along for the ride.

conservancy_beancount/data.py

@@ -264,6 +264,27 @@ class Posting(BasePosting):
     # If it did, this declaration would pass without issue.
     meta: Metadata  # type:ignore[assignment]
 
+    @classmethod
+    def from_beancount(cls,
+                       txn: Transaction,
+                       index: int,
+                       post: Optional[BasePosting]=None,
+    ) -> 'Posting':
+        if post is None:
+            post = txn.postings[index]
+        return cls(
+            Account(post.account),
+            *post[1:5],
+            # see rationale above about Posting.meta
+            PostingMeta(txn, index, post), # type:ignore[arg-type]
+        )
+
+    @classmethod
+    def from_txn(cls, txn: Transaction) -> Iterable['Posting']:
+        """Yield an enhanced Posting object for every posting in the transaction"""
+        for index, post in enumerate(txn.postings):
+            yield cls.from_beancount(txn, index, post)
+
 
 def balance_of(txn: Transaction,
                *preds: Callable[[Account], Optional[bool]],
@@ -277,7 +298,7 @@ def balance_of(txn: Transaction,
     balance_of uses the "weight" of each posting, so the return value will
     use the currency of the postings' cost when available.
     """
-    match_posts = [post for post in iter_postings(txn)
+    match_posts = [post for post in Posting.from_txn(txn)
                    if any(pred(post.account) for pred in preds)]
     number = decimal.Decimal(0)
     if not match_posts:
@@ -299,13 +320,3 @@ def is_opening_balance_txn(txn: Transaction) -> bool:
     if not rest.currency:
         return False
     return abs(opening_equity.number + rest.number) < decimal.Decimal('.01')
-
-def iter_postings(txn: Transaction) -> Iterator[Posting]:
-    """Yield an enhanced Posting object for every posting in the transaction"""
-    for index, source in enumerate(txn.postings):
-        yield Posting(
-            Account(source.account),
-            *source[1:5],
-            # see rationale above about Posting.meta
-            PostingMeta(txn, index, source), # type:ignore[arg-type]
-        )

conservancy_beancount/plugin/core.py

@@ -195,7 +195,7 @@ class _PostingHook(TransactionHook, metaclass=abc.ABCMeta):
 
     def run(self, txn: Transaction) -> errormod.Iter:
         if self._run_on_txn(txn):
-            for post in data.iter_postings(txn):
+            for post in data.Posting.from_txn(txn):
                 if self._run_on_post(txn, post):
                     yield from self.post_run(txn, post)
 

conservancy_beancount/plugin/meta_entity.py

@@ -60,7 +60,7 @@ class MetaEntity(core.TransactionHook):
             txn_entity_ok = False
         if txn_entity_ok is False:
             yield errormod.InvalidMetadataError(txn, self.METADATA_KEY, txn_entity)
-        for post in data.iter_postings(txn):
+        for post in data.Posting.from_txn(txn):
             if not post.account.is_under(
                     'Assets:Receivable',
                     'Expenses',

tests/test_data_posting.py

@@ -1,4 +1,4 @@
-"""Test data.iter_postings function"""
+"""Test Posting methods"""
 # Copyright © 2020  Brett Smith
 #
 # This program is free software: you can redistribute it and/or modify
@@ -27,20 +27,33 @@ def simple_txn(index=None, key=None):
         ('Income:Donations', -5, {'note': 'donation love', 'extra': 'Extra'}),
     ])
 
-def test_iter_postings(simple_txn):
-    for source, post in zip(simple_txn.postings, data.iter_postings(simple_txn)):
+def test_from_beancount():
+    txn = testutil.Transaction(payee='Smith-Dakota', postings=[
+        ('Income:Donations', -50),
+        ('Assets:Cash', 50, {'receipt': 'cash-donation.pdf'}),
+    ])
+    post = data.Posting.from_beancount(txn, 1)
+    # We don't just want to assert isinstance(post.attr, data.SomeClass);
+    # we also want to double-check that attributes were instantiated correctly.
+    assert post.account.is_under('Assets:Cash')
+    assert post.meta['receipt'] == 'cash-donation.pdf'
+    assert post.meta['entity'] == 'Smith-Dakota'
+    assert post.meta.date == testutil.FY_MID_DATE
+
+def test_setting_metadata_propagates_to_source(simple_txn):
+    src_post = simple_txn.postings[1]
+    post = data.Posting.from_beancount(simple_txn, 1)
+    post.meta['edited'] = 'yes'
+    assert src_post.meta['edited'] == 'yes'
+    assert not isinstance(src_post.meta, data.PostingMeta)
+
+def test_deleting_metadata_propagates_to_source(simple_txn):
+    post = data.Posting.from_beancount(simple_txn, 1)
+    del post.meta['extra']
+    assert 'extra' not in simple_txn.postings[1].meta
+
+def test_from_txn(simple_txn):
+    for source, post in zip(simple_txn.postings, data.Posting.from_txn(simple_txn)):
         assert all(source[x] == post[x] for x in range(len(source) - 1))
         assert isinstance(post.account, data.Account)
         assert post.meta['note']  # Only works with PostingMeta
-
-def test_setting_metadata_propagates_to_source(simple_txn):
-    for index, post in enumerate(data.iter_postings(simple_txn)):
-        post.meta['edited'] = str(index)
-    for index, post in enumerate(simple_txn.postings):
-        assert post.meta['edited'] == str(index)
-        assert not isinstance(post.meta, data.PostingMeta)
-
-def test_deleting_metadata_propagates_to_source(simple_txn):
-    posts = list(data.iter_postings(simple_txn))
-    del posts[1].meta['extra']
-    assert 'extra' not in simple_txn.postings[1].meta