Make the sentinels restore the original object after pickling

docs/code_examples/outputs/docs_ex09_annotated.output

@@ -1,12 +1,12 @@
 <generated class X; x='Value of x', a='Not In __init__ signature', c=[], e='Value of e', f=42> 
 
-{'x': Attribute(default=<NOTHING OBJECT>, default_factory=<NOTHING OBJECT>, type=<class 'str'>, doc=None, init=True, repr=True, compare=True, kw_only=False, iter=True, serialize=True, metadata={}),
- 'a': Attribute(default='Not In __init__ signature', default_factory=<NOTHING OBJECT>, type=<class 'int'>, doc=None, init=False, repr=True, compare=True, kw_only=False, iter=True, serialize=True, metadata={}),
- 'b': Attribute(default='Not In Repr', default_factory=<NOTHING OBJECT>, type=<class 'str'>, doc=None, init=True, repr=False, compare=True, kw_only=False, iter=True, serialize=True, metadata={}),
- 'c': Attribute(default=<NOTHING OBJECT>, default_factory=<class 'list'>, type=list[str], doc=None, init=True, repr=True, compare=False, kw_only=False, iter=True, serialize=True, metadata={}),
- 'd': Attribute(default='Not Anywhere', default_factory=<NOTHING OBJECT>, type=<class 'str'>, doc=None, init=False, repr=False, compare=False, kw_only=False, iter=True, serialize=True, metadata={}),
- 'e': Attribute(default=<NOTHING OBJECT>, default_factory=<NOTHING OBJECT>, type=<class 'str'>, doc=None, init=True, repr=True, compare=False, kw_only=True, iter=True, serialize=True, metadata={}),
- 'f': Attribute(default=42, default_factory=<NOTHING OBJECT>, type=ForwardRef('unknown'), doc=None, init=True, repr=True, compare=False, kw_only=False, iter=True, serialize=False, metadata={})}
+{'x': Attribute(default=<NOTHING Sentinel>, default_factory=<NOTHING Sentinel>, type=<class 'str'>, doc=None, init=True, repr=True, compare=True, kw_only=False, iter=True, serialize=True, metadata={}),
+ 'a': Attribute(default='Not In __init__ signature', default_factory=<NOTHING Sentinel>, type=<class 'int'>, doc=None, init=False, repr=True, compare=True, kw_only=False, iter=True, serialize=True, metadata={}),
+ 'b': Attribute(default='Not In Repr', default_factory=<NOTHING Sentinel>, type=<class 'str'>, doc=None, init=True, repr=False, compare=True, kw_only=False, iter=True, serialize=True, metadata={}),
+ 'c': Attribute(default=<NOTHING Sentinel>, default_factory=<class 'list'>, type=list[str], doc=None, init=True, repr=True, compare=False, kw_only=False, iter=True, serialize=True, metadata={}),
+ 'd': Attribute(default='Not Anywhere', default_factory=<NOTHING Sentinel>, type=<class 'str'>, doc=None, init=False, repr=False, compare=False, kw_only=False, iter=True, serialize=True, metadata={}),
+ 'e': Attribute(default=<NOTHING Sentinel>, default_factory=<NOTHING Sentinel>, type=<class 'str'>, doc=None, init=True, repr=True, compare=False, kw_only=True, iter=True, serialize=True, metadata={}),
+ 'f': Attribute(default=42, default_factory=<NOTHING Sentinel>, type=ForwardRef('unknown'), doc=None, init=True, repr=True, compare=False, kw_only=False, iter=True, serialize=False, metadata={})}
 
 
 <generated class Y; x='Value of x', a='Not In __init__ signature', c=[], e='Value of e', f=42> 

docs/code_examples/outputs/tutorial_code.output

@@ -1,6 +1,6 @@
-({'field_1': CustomField(default='First Field', default_factory=<NOTHING OBJECT>, type=<NOTHING OBJECT>, doc=None, init=True, repr=True, compare=True, kw_only=False, report=True),
-  'field_2': CustomField(default='Second Field', default_factory=<NOTHING OBJECT>, type=<NOTHING OBJECT>, doc=None, init=True, repr=True, compare=True, kw_only=False, report=True),
-  'field_3': CustomField(default='Third Field', default_factory=<NOTHING OBJECT>, type=<NOTHING OBJECT>, doc=None, init=True, repr=True, compare=True, kw_only=False, report=False)},
+({'field_1': CustomField(default='First Field', default_factory=<NOTHING Sentinel>, type=<NOTHING Sentinel>, doc=None, init=True, repr=True, compare=True, kw_only=False, report=True),
+  'field_2': CustomField(default='Second Field', default_factory=<NOTHING Sentinel>, type=<NOTHING Sentinel>, doc=None, init=True, repr=True, compare=True, kw_only=False, report=True),
+  'field_3': CustomField(default='Third Field', default_factory=<NOTHING Sentinel>, type=<NOTHING Sentinel>, doc=None, init=True, repr=True, compare=True, kw_only=False, report=False)},
  {'__fields__': None})
 @property
 def report(self):

pyproject.toml

@@ -83,3 +83,7 @@ exclude_also = [
 # A combination of types in stubs and tests using dataclass syntax
 # means that there are a number of annotations in otherwise unannotated areas
 disable_error_code = ["annotation-unchecked"]
+
+[tool.uv]
+exclude-newer = "1 week"
+exclude-newer-package = { reannotate = "1 hour" }

src/ducktools/classbuilder/__init__.py

@@ -175,12 +175,24 @@ def build_completed(cls):
 # As 'None' can be a meaningful value we need a sentinel value
 # to use to show no value has been provided.
 class _NothingType:
-    def __init__(self, custom=None):
-        self.custom = custom
+    # Repeated calls to the same nothing type should
+    # return the same object
+    _registry = {}  # type: ignore
+
+    def __new__(cls, custom=None):
+        # Instances with the same custom name
+        # should be the same object
+        inst = cls._registry.get(custom)
+        if not inst:
+            inst = super().__new__(cls)
+            inst.custom = custom
+            cls._registry[custom] = inst
+        return inst
+
     def __repr__(self):
         if self.custom:
-            return f"<{self.custom} NOTHING OBJECT>"
-        return "<NOTHING OBJECT>"
+            return f"<{self.custom} NOTHING Sentinel>"
+        return "<NOTHING Sentinel>"
 
 
 NOTHING = _NothingType()

src/ducktools/classbuilder/__init__.pyi

@@ -47,7 +47,7 @@ def build_completed(cls: type) -> bool: ...
 def _get_inst_fields(inst: typing.Any) -> dict[str, typing.Any]: ...
 
 class _NothingType:
-    def __init__(self, custom: str | None = ...) -> None: ...
+    def __new__(cls, custom: str | None = ...) -> typing.Self: ...
     def __repr__(self) -> str: ...
 NOTHING: _NothingType
 FIELD_NOTHING: _NothingType

tests/test_core.py

@@ -1,10 +1,14 @@
 # Tests for the core 'builder'
 import inspect
 import pytest
+import pickle
 
 from ducktools.classbuilder import (
+    _NothingType,
+
     INTERNALS_DICT,
     NOTHING,
+    FIELD_NOTHING,
 
     add_methods,
     builder,
@@ -124,6 +128,16 @@ def test_construct_field():
         Field(default=None, default_factory=list)
 
 
+def test_nothing_type():
+    # Test that creating new sentinels actually
+    # gives the original sentinel
+    assert _NothingType() is NOTHING
+    assert _NothingType("FIELD") is FIELD_NOTHING
+
+    dumps, loads = pickle.dumps, pickle.loads
+    assert loads(dumps(NOTHING)) is NOTHING
+
+
 def test_eq_field():
     f1 = Field(default=True)
     f2 = Field(default=False)
@@ -537,10 +551,10 @@ class Ex:
     assert repr(flds).endswith(
         "GatheredFields("
         "fields={'x': Field("
-        "default=1, default_factory=<NOTHING OBJECT>, type=<NOTHING OBJECT>, doc=None, "
+        "default=1, default_factory=<NOTHING Sentinel>, type=<NOTHING Sentinel>, doc=None, "
         "init=True, repr=True, compare=True, kw_only=False"
         ")}, "
-        "modifications={'x': <NOTHING OBJECT>}"
+        "modifications={'x': <NOTHING Sentinel>}"
         ")"
     )