improvement: Mark some more settings as frozen or add unsafe hash functions to support passing them as metadata to typing_extensions.Annotated in Python 3.6

Author: NiklasRosenstein

databind.core/.changelog/_unreleased.toml

@@ -9,3 +9,9 @@ id = "78606f4e-7b4c-4f88-9143-e40b12d86100"
 type = "improvement"
 description = "Expose public API in `databind.core` root module."
 author = "@NiklasRosenstein"
+
+[[entries]]
+id = "8d5cd812-ba8f-4c4d-9681-b0d8866ae023"
+type = "improvement"
+description = "Mark some more settings as frozen or add unsafe hash functions to support passing them as metadata to `typing_extensions.Annotated` in Python 3.6"
+author = "@NiklasRosenstein"

databind.core/src/databind/core/settings.py

@@ -326,7 +326,7 @@ class SerializeDefaults(BooleanSetting):
     to how the name of the setting appears assertive of the fact that the instance indicates the setting is enabled."""
 
 
-@dataclasses.dataclass
+@dataclasses.dataclass(frozen=True)
 class DeserializeAs(Setting):
     """Indicates that a field should be deserialized as the given type instead of the type of the field. This is
     typically used when a field should be typed as an abstract class or interface, but during deserialization of the
@@ -360,7 +360,7 @@ class MyClass:
     priority: Priority = Priority.NORMAL
 
 
-@dataclasses.dataclass
+@dataclasses.dataclass(frozen=True)
 class Precision(Setting):
     """A setting to describe the precision for #decimal.Decimal fields."""
 
@@ -530,7 +530,7 @@ def import_() -> "ImportUnionMembers":
         return ImportUnionMembers()
 
 
-@dataclasses.dataclass
+@dataclasses.dataclass(init=False, unsafe_hash=True)
 class DateFormat(Setting):
     """The #DateFormat setting is used to describe the date format to use for #datetime.datetime, #datetime.date
     and #datetime.time values when formatting them as a string, i.e. usually when the date/time is serialized, and

docs/content/core/settings.md

@@ -29,3 +29,8 @@ unless specified otherwise. The following priority groups exist:
 * `ULTIMATE`: Settings with this priority group are resolved before `HIGH` settings.
 
 Converters are usually only interested in the first instance of any setting type.
+
+## Notes about implementing custom settings
+
+To support Python 3.6, `typing_extensions.Annotated` metadata must be hashable. As such, settings that you want users
+to be able to pass into annotated type hints in Python 3.6 must support hashing (identity hashing is often sufficient).