data: Add Metadata class.

As I start writing more link-checking hooks, I want a common place to
write link-parsing code.  This new class will be that place.

conservancy_beancount/data.py

@@ -19,7 +19,7 @@ throughout Conservancy tools.
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-import collections.abc
+import collections
 
 from beancount.core import account as bc_account
 
@@ -28,6 +28,7 @@ from typing import (
     Iterator,
     MutableMapping,
     Optional,
+    Sequence,
 )
 
 from .beancount_types import (
@@ -98,7 +99,45 @@ class Account(str):
         return None
 
 
-class PostingMeta(collections.abc.MutableMapping):
+class Metadata(MutableMapping[MetaKey, MetaValue]):
+    """Transaction or posting metadata
+
+    This class wraps a Beancount metadata dictionary with additional methods
+    for common parsing and query tasks.
+    """
+
+    def __init__(self, source: MutableMapping[MetaKey, MetaValue]) -> None:
+        self.meta = source
+
+    def __iter__(self) -> Iterator[MetaKey]:
+        return iter(self.meta)
+
+    def __len__(self) -> int:
+        return len(self.meta)
+
+    def __getitem__(self, key: MetaKey) -> MetaValue:
+        return self.meta[key]
+
+    def __setitem__(self, key: MetaKey, value: MetaValue) -> None:
+        self.meta[key] = value
+
+    def __delitem__(self, key: MetaKey) -> None:
+        del self.meta[key]
+
+    def get_links(self, key: MetaKey) -> Sequence[str]:
+        try:
+            value = self.meta[key]
+        except KeyError:
+            return ()
+        if isinstance(value, str):
+            return value.split()
+        else:
+            raise TypeError("{} metadata is a {}, not str".format(
+                key, type(value).__name__,
+            ))
+
+
+class PostingMeta(Metadata):
     """Combined access to posting metadata with its parent transaction metadata
 
     This lets you access posting metadata through a single dict-like object.
@@ -127,38 +166,26 @@ class PostingMeta(collections.abc.MutableMapping):
         self.txn = txn
         self.index = index
         self.post = post
-
+        if post.meta is None:
-    def __iter__(self) -> Iterator[MetaKey]:
+            self.meta = self.txn.meta
-        keys: Iterable[MetaKey]
-        if self.post.meta is None:
-            keys = self.txn.meta.keys()
         else:
-            keys = frozenset(self.post.meta.keys()).union(self.txn.meta.keys())
+            self.meta = collections.ChainMap(post.meta, txn.meta)
-        return iter(keys)
-
-    def __len__(self) -> int:
-        return sum(1 for _ in self)
-
-    def __getitem__(self, key: MetaKey) -> MetaValue:
-        if self.post.meta:
-            try:
-                return self.post.meta[key]
-            except KeyError:
-                pass
-        return self.txn.meta[key]
 
     def __setitem__(self, key: MetaKey, value: MetaValue) -> None:
         if self.post.meta is None:
             self.post = self.post._replace(meta={key: value})
             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]
         else:
-            self.post.meta[key] = value
+            super().__setitem__(key, value)
 
     def __delitem__(self, key: MetaKey) -> None:
         if self.post.meta is None:
             raise KeyError(key)
         else:
-            del self.post.meta[key]
+            super().__delitem__(key)
 
 
 class Posting(BasePosting):
@@ -178,7 +205,7 @@ class Posting(BasePosting):
     # declaration should also use MutableMapping, because it would be very
     # unusual for code to specifically require a Dict over that.
     # If it did, this declaration would pass without issue.
-    meta: MutableMapping[MetaKey, MetaValue]  # type:ignore[assignment]
+    meta: Metadata  # type:ignore[assignment]
 
 
 def iter_postings(txn: Transaction) -> Iterator[Posting]:

tests/test_data_metadata.py

@@ -0,0 +1,57 @@
+"""Test Metadata class"""
+# Copyright © 2020  Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+import pytest
+
+from . import testutil
+
+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'}),
+    ])
+SIMPLE_TXN_METAKEYS = frozenset(['filename', 'lineno', 'note'])
+
+def test_metadata_transforms_source():
+    source = {'1': 'one'}
+    meta = data.Metadata(source)
+    meta['2'] = 'two'
+    assert source['2'] == 'two'
+    del meta['1']
+    assert set(source) == {'2'}
+
+@pytest.mark.parametrize('value', [
+    '',
+    'link',
+    'link1  link2',
+    ' link1  link2   link3    ',
+])
+def test_get_links(value):
+    meta = data.Metadata({'key': value})
+    assert list(meta.get_links('key')) == value.split()
+
+def test_get_links_missing():
+    meta = data.Metadata({})
+    assert not meta.get_links('key')
+
+@pytest.mark.parametrize('value', testutil.NON_STRING_METADATA_VALUES)
+def test_get_links_bad_type(value):
+    meta = data.Metadata({'key': value})
+    with pytest.raises(TypeError):
+        meta.get_links('key')

tests/test_data_posting_meta.py

@@ -80,6 +80,14 @@ def test_iter_with_empty_post_meta(simple_txn):
 def test_iter_with_post_meta_over_txn(simple_txn):
     assert set(data.PostingMeta(simple_txn, 1)) == SIMPLE_TXN_METAKEYS.union(['extra'])
 
+def test_get_links_from_txn(simple_txn):
+    meta = data.PostingMeta(simple_txn, 0)
+    assert list(meta.get_links('note')) == ['txn', 'note']
+
+def test_get_links_from_post_override(simple_txn):
+    meta = data.PostingMeta(simple_txn, 1)
+    assert list(meta.get_links('note')) == ['donation', 'love']
+
 # 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):

tests/testutil.py

@@ -54,6 +54,9 @@ def test_path(s):
         s = TESTS_DIR / s
     return s
 
+def Amount(number, currency='USD'):
+    return bc_amount.Amount(Decimal(number), currency)
+
 def Posting(account, number,
             currency='USD', cost=None, price=None, flag=None,
             **meta):
@@ -68,6 +71,13 @@ def Posting(account, number,
         meta,
     )
 
+NON_STRING_METADATA_VALUES = [
+    Decimal(5),
+    FY_MID_DATE,
+    Amount(50),
+    Amount(500, None),
+]
+
 class Transaction:
     def __init__(self,
                  date=FY_MID_DATE, flag='*', payee=None,