self.post.meta[key] = value

    def __delitem__(self, key: MetaKey) -> None:

        if self.post.meta is None:

            raise KeyError(key)

        else:

            del self.post.meta[key]

def iter_postings(txn: Transaction) -> Iterator[Posting]:

    for index, source in enumerate(txn.postings):

        yield source._replace(

            meta=PostingMeta(txn, index, source),

        )

from conservancy_beancount import data

@pytest.fixture

def simple_txn(index=None, key=None):

    return testutil.Transaction(note='txn note', postings=[

        ('Assets:Cash', 5),

        ('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)):

        assert all(source[x] == post[x] for x in range(len(source) - 1))

        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']