Make MISSING usable in type annotations, and add EMPTY and SENTINEL variations.

Author: matthewwardrop

spec_classes/__init__.py

@@ -1,7 +1,16 @@
 from ._version import __version__, __version_tuple__
 from .errors import FrozenInstanceError
 from .spec_class import spec_class
-from .types import Alias, DeprecatedAlias, Attr, AttrProxy, MISSING, spec_property
+from .types import (
+    spec_property,
+    Alias,
+    DeprecatedAlias,
+    Attr,
+    AttrProxy,
+    MISSING,
+    EMPTY,
+    SENTINEL,
+)
 
 __author__ = "Matthew Wardrop"
 __author_email__ = "mpwardrop@gmail.com"
@@ -12,11 +21,13 @@
     "__author__",
     "__author_email__",
     "spec_class",
+    "spec_property",
     "FrozenInstanceError",
-    "MISSING",
     "Alias",
     "DeprecatedAlias",
     "Attr",
     "AttrProxy",
-    "spec_property",
+    "MISSING",
+    "EMPTY",
+    "SENTINEL",
 ]

spec_classes/types/__init__.py

@@ -2,7 +2,7 @@
 from .attr import Attr
 from .attr_proxy import AttrProxy
 from .keyed import KeyedList, KeyedSet
-from .missing import MISSING
+from .missing import MISSING, EMPTY, SENTINEL
 from .spec_property import spec_property
 from .validated import ValidatedType, bounded, validated
 
@@ -14,6 +14,8 @@
     "KeyedList",
     "KeyedSet",
     "MISSING",
+    "EMPTY",
+    "SENTINEL",
     "ValidatedType",
     "bounded",
     "spec_property",

spec_classes/types/missing.py

@@ -1,23 +1,36 @@
 # Sentinel for unset inputs to spec_class methods
-class _MissingType:
-    __instance__ = None
 
-    def __new__(cls):
-        if cls.__instance__ is None:
-            cls.__instance__ = super(_MissingType, cls).__new__(cls)
-        return cls.__instance__
 
-    def __bool__(self):
+class _MissingType(type):
+    """
+    This metaclass is used to create singleton falsey classes for use as missing
+    and/or sentinel placeholder values.
+    """
+
+    def __repr__(cls):
+        return cls.__name__
+
+    def __bool__(cls):
         return False
 
-    def __repr__(self):
-        return "MISSING"
+    def __call__(cls):
+        return cls
+
+
+class MISSING(metaclass=_MissingType):
+    """
+    Used to represent attributes that have not yet been assigned a value.
+    """
 
-    def __copy__(self):
-        return self
 
-    def __deepcopy__(self, memo):
-        return self
+class EMPTY(metaclass=_MissingType):
+    """
+    Used to represent empty arguments in methods/etc, which is useful when it is
+    important to distinguish between empty and "MISSING" values.
+    """
 
 
-MISSING = _MissingType()
+class SENTINEL(metaclass=_MissingType):
+    """
+    A generic sentinel that can be used to check fallthrough conditions.
+    """

tests/types/test_missing.py

@@ -1,11 +1,13 @@
 import copy
 
-from spec_classes.types.missing import _MissingType, MISSING
+from spec_classes.types.missing import MISSING, EMPTY, SENTINEL
 
 
 def test_missing():
-    assert MISSING is _MissingType()
+    assert MISSING != EMPTY
+    assert MISSING != SENTINEL
     assert bool(MISSING) is False
     assert repr(MISSING) == "MISSING"
     assert copy.copy(MISSING) is MISSING
     assert copy.deepcopy(MISSING) is MISSING
+    assert MISSING() is MISSING