plugin: Refactor _RequireLinksPostingMetadataHook out of MetaInvoice.

This will be a common hook type.

conservancy_beancount/plugin/core.py

@@ -239,3 +239,25 @@ class _NormalizePostingMetadataHook(_PostingHook):
             post.meta[self.METADATA_KEY] = set_value
         else:
             yield error
+
+
+class _RequireLinksPostingMetadataHook(_PostingHook):
+    """Base class to require that posting metadata include links"""
+    # This base class confirms that a posting's metadata has one or more links
+    # under METADATA_KEY.
+    # Most subclasses only need to define METADATA_KEY and _run_on_post.
+    METADATA_KEY: str
+
+    def __init_subclass__(cls) -> None:
+        super().__init_subclass__()
+        cls.HOOK_GROUPS = cls.HOOK_GROUPS.union(['metadata', cls.METADATA_KEY])
+
+    def post_run(self, txn: Transaction, post: data.Posting) -> errormod.Iter:
+        try:
+            problem = not post.meta.get_links(self.METADATA_KEY)
+            value = None
+        except TypeError:
+            problem = True
+            value = post.meta[self.METADATA_KEY]
+        if problem:
+            yield errormod.InvalidMetadataError(txn, self.METADATA_KEY, value, post)

conservancy_beancount/plugin/meta_invoice.py

@@ -22,19 +22,8 @@ from ..beancount_types import (
     Transaction,
 )
 
-class MetaInvoice(core._PostingHook):
+class MetaInvoice(core._RequireLinksPostingMetadataHook):
     METADATA_KEY = 'invoice'
-    HOOK_GROUPS = frozenset(['metadata', METADATA_KEY])
 
     def _run_on_post(self, txn: Transaction, post: data.Posting) -> bool:
         return post.account.is_under('Accrued') is not None
-
-    def post_run(self, txn: Transaction, post: data.Posting) -> errormod.Iter:
-        try:
-            problem = not post.meta.get_links(self.METADATA_KEY)
-            value = None
-        except TypeError:
-            problem = True
-            value = post.meta[self.METADATA_KEY]
-        if problem:
-            yield errormod.InvalidMetadataError(txn, self.METADATA_KEY, value, post)