Add support for cached_properties to slotted attrs classes. (#1200)

* Add support for cached_properties to slotted attrs classes.

* Remove locking from implementation

* Add test for multiple cached properties and fix bug

* Add changelog file

* Document slotted cached properties

* Add cached_property hypothesis check.

* Only run cached_property imports on python 3.8+

* Use cached _obj_setattr instead of `object.__setattr__`

* Correctly resolve mro for __getattr__ in cached properties

* Use _get_annotations rather than branching on class dict entry

* Optimise __getattr__ code by front loading branching, and injecting locasl variables

* Remove unnecessary `__attrs_original_getattr__` from class dictionary.

---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/1200.change.md

@@ -0,0 +1 @@
+Slotted classes now transform `functools.cached_property` decorated methods to support equivalent semantics.

docs/how-does-it-work.md

@@ -96,3 +96,23 @@ Pick what's more important to you.
 You should avoid instantiating lots of frozen slotted classes (i.e. `@frozen`) in performance-critical code.
 
 Frozen dict classes have barely a performance impact, unfrozen slotted classes are even *faster* than unfrozen dict classes (i.e. regular classes).
+
+
+(how-slotted-cached_property)=
+
+## Cached Properties on Slotted Classes.
+
+By default, the standard library `functools.cached_property` decorator does not work on slotted classes,
+because it requires a `__dict__` to store the cached value.
+This could be surprising when uses *attrs*, as makes using slotted classes so easy,
+so attrs will convert `functools.cached_property` decorated methods, when constructing slotted classes.
+
+Getting this working is achieved by:
+* Adding names to `__slots__` for the wrapped methods.
+* Adding a `__getattr__` method to set values on the wrapped methods.
+
+For most users this should mean that it works transparently.
+
+Note that the implementation does not guarantee that the wrapped method is called
+only once in multi-threaded usage.  This matches the implementation of `cached_property`
+in python v3.12.

src/attr/_compat.py

@@ -10,6 +10,7 @@
 
 
 PYPY = platform.python_implementation() == "PyPy"
+PY_3_8_PLUS = sys.version_info[:2] >= (3, 8)
 PY_3_9_PLUS = sys.version_info[:2] >= (3, 9)
 PY310 = sys.version_info[:2] >= (3, 10)
 PY_3_12_PLUS = sys.version_info[:2] >= (3, 12)

src/attr/_make.py

@@ -3,7 +3,9 @@
 import contextlib
 import copy
 import enum
+import functools
 import inspect
+import itertools
 import linecache
 import sys
 import types
@@ -16,6 +18,7 @@
 from . import _compat, _config, setters
 from ._compat import (
     PY310,
+    PY_3_8_PLUS,
     _AnnotationExtractor,
     get_generic_base,
 )
@@ -597,6 +600,62 @@ def _transform_attrs(
     return _Attributes((AttrsClass(attrs), base_attrs, base_attr_map))
 
 
+def _make_cached_property_getattr(
+    cached_properties,
+    original_getattr,
+    cls,
+):
+    lines = [
+        # Wrapped to get `__class__` into closure cell for super()
+        # (It will be replaced with the newly constructed class after construction).
+        "def wrapper():",
+        "    __class__ = _cls",
+        "    def __getattr__(self, item, cached_properties=cached_properties, original_getattr=original_getattr, _cached_setattr_get=_cached_setattr_get):",
+        "         func = cached_properties.get(item)",
+        "         if func is not None:",
+        "              result = func(self)",
+        "              _setter = _cached_setattr_get(self)",
+        "              _setter(item, result)",
+        "              return result",
+    ]
+    if original_getattr is not None:
+        lines.append(
+            "         return original_getattr(self, item)",
+        )
+    else:
+        lines.extend(
+            [
+                "         if hasattr(super(), '__getattr__'):",
+                "              return super().__getattr__(item)",
+                "         original_error = f\"'{self.__class__.__name__}' object has no attribute '{item}'\"",
+                "         raise AttributeError(original_error)",
+            ]
+        )
+
+    lines.extend(
+        [
+            "    return __getattr__",
+            "__getattr__ = wrapper()",
+        ]
+    )
+
+    unique_filename = _generate_unique_filename(cls, "getattr")
+
+    glob = {
+        "cached_properties": cached_properties,
+        "_cached_setattr_get": _obj_setattr.__get__,
+        "_cls": cls,
+        "original_getattr": original_getattr,
+    }
+
+    return _make_method(
+        "__getattr__",
+        "\n".join(lines),
+        unique_filename,
+        glob,
+    )
+
+
 def _frozen_setattrs(self, name, value):
     """
     Attached to frozen classes as __setattr__.
@@ -857,9 +916,46 @@ def _create_slots_class(self):
         ):
             names += ("__weakref__",)
 
+        if PY_3_8_PLUS:
+            cached_properties = {
+                name: cached_property.func
+                for name, cached_property in cd.items()
+                if isinstance(cached_property, functools.cached_property)
+            }
+        else:
+            # `functools.cached_property` was introduced in 3.8.
+            # So can't be used before this.
+            cached_properties = {}
+
+        # Collect methods with a `__class__` reference that are shadowed in the new class.
+        # To know to update them.
+        additional_closure_functions_to_update = []
+        if cached_properties:
+            # Add cached properties to names for slotting.
+            names += tuple(cached_properties.keys())
+
+            for name in cached_properties:
+                # Clear out function from class to avoid clashing.
+                del cd[name]
+
+            class_annotations = _get_annotations(self._cls)
+            for name, func in cached_properties.items():
+                annotation = inspect.signature(func).return_annotation
+                if annotation is not inspect.Parameter.empty:
+                    class_annotations[name] = annotation
+
+            original_getattr = cd.get("__getattr__")
+            if original_getattr is not None:
+                additional_closure_functions_to_update.append(original_getattr)
+
+            cd["__getattr__"] = _make_cached_property_getattr(
+                cached_properties, original_getattr, self._cls
+            )
+
         # We only add the names of attributes that aren't inherited.
         # Setting __slots__ to inherited attributes wastes memory.
         slot_names = [name for name in names if name not in base_names]
+
         # There are slots for attributes from current class
         # that are defined in parent classes.
         # As their descriptors may be overridden by a child class,
@@ -873,6 +969,7 @@ def _create_slots_class(self):
         cd.update(reused_slots)
         if self._cache_hash:
             slot_names.append(_hash_cache_field)
+
         cd["__slots__"] = tuple(slot_names)
 
         cd["__qualname__"] = self._cls.__qualname__
@@ -886,7 +983,9 @@ def _create_slots_class(self):
         # compiler will bake a reference to the class in the method itself
         # as `method.__closure__`.  Since we replace the class with a
         # clone, we rewrite these references so it keeps working.
-        for item in cls.__dict__.values():
+        for item in itertools.chain(
+            cls.__dict__.values(), additional_closure_functions_to_update
+        ):
             if isinstance(item, (classmethod, staticmethod)):
                 # Class- and staticmethods hide their functions inside.
                 # These might need to be rewritten as well.
@@ -909,7 +1008,6 @@ def _create_slots_class(self):
                 else:
                     if match:
                         cell.cell_contents = cls
-
         return cls
 
     def add_repr(self, ns):

tests/strategies.py

@@ -3,7 +3,7 @@
 """
 Testing strategies for Hypothesis-based tests.
 """
-
+import functools
 import keyword
 import string
 
@@ -13,6 +13,8 @@
 
 import attr
 
+from attr._compat import PY_3_8_PLUS
+
 from .utils import make_class
 
 
@@ -111,13 +113,19 @@ def simple_attrs_with_metadata(draw):
 
 simple_attrs = simple_attrs_without_metadata | simple_attrs_with_metadata()
 
+
 # Python functions support up to 255 arguments.
 list_of_attrs = st.lists(simple_attrs, max_size=3)
 
 
 @st.composite
 def simple_classes(
-    draw, slots=None, frozen=None, weakref_slot=None, private_attrs=None
+    draw,
+    slots=None,
+    frozen=None,
+    weakref_slot=None,
+    private_attrs=None,
+    cached_property=None,
 ):
     """
     A strategy that generates classes with default non-attr attributes.
@@ -157,6 +165,7 @@ class HypClass:
     pre_init_flag = draw(st.booleans())
     post_init_flag = draw(st.booleans())
     init_flag = draw(st.booleans())
+    cached_property_flag = draw(st.booleans())
 
     if pre_init_flag:
 
@@ -179,9 +188,22 @@ def init(self, *args, **kwargs):
 
         cls_dict["__init__"] = init
 
+    bases = (object,)
+    if cached_property or (
+        PY_3_8_PLUS and cached_property is None and cached_property_flag
+    ):
+
+        class BaseWithCachedProperty:
+            @functools.cached_property
+            def _cached_property(self) -> int:
+                return 1
+
+        bases = (BaseWithCachedProperty,)
+
     return make_class(
         "HypClass",
         cls_dict,
+        bases=bases,
         slots=slots_flag if slots is None else slots,
         frozen=frozen_flag if frozen is None else frozen,
         weakref_slot=weakref_flag if weakref_slot is None else weakref_slot,

tests/test_3rd_party.py

@@ -19,7 +19,7 @@ class TestCloudpickleCompat:
     Tests for compatibility with ``cloudpickle``.
     """
 
-    @given(simple_classes())
+    @given(simple_classes(cached_property=False))
     def test_repr(self, cls):
         """
         attrs instances can be pickled and un-pickled with cloudpickle.