Annotate inferred column types with arbitrary runtime notes (metadata)

Similar to pytest's marks or Python's Exception's notes, we can add arbitrary data classes and retrieve the latest note (marker). This makes the customization of column types easier without overriding the whole hierarchy of column types.

Author: Sergey Vasilyev

data_diff/abcs/database_types.py

@@ -1,6 +1,6 @@
 import decimal
 from abc import ABC, abstractmethod
-from typing import Tuple, Union
+from typing import List, Optional, Tuple, Type, TypeVar, Union
 from datetime import datetime
 
 import attrs
@@ -12,9 +12,25 @@
 DbKey = Union[int, str, bytes, ArithUUID, ArithAlphanumeric]
 DbTime = datetime
 
+N = TypeVar('N')
 
-@attrs.define(frozen=True)
+
+@attrs.define(frozen=True, kw_only=True)
 class ColType:
+
+    # Arbitrary metadata added and fetched at runtime.
+    _notes: List[N] = attrs.field(factory=list, init=False, hash=False)
+
+    def add_note(self, note: N) -> None:
+        self._notes.append(note)
+
+    def get_note(self, cls: Type[N]) -> Optional[N]:
+        """Get the latest added note of type ``cls`` or its descendants."""
+        for note in reversed(self._notes):
+            if isinstance(note, cls):
+                return note
+        return None
+
     @property
     def supported(self) -> bool:
         return True