Add support for cached_properties to slotted attrs classes.

Author: Danny Cooper

src/attr/_make.py

@@ -3,12 +3,14 @@
 import contextlib
 import copy
 import enum
+import functools
 import inspect
 import linecache
 import sys
 import types
 import typing
 
+from _thread import RLock
 from operator import itemgetter
 
 # We need to import _compat itself in addition to the _compat members to avoid
@@ -598,6 +600,28 @@ def _transform_attrs(
     return _Attributes((AttrsClass(attrs), base_attrs, base_attr_map))
 
 
+def _make_cached_property_getattr(cached_properties, original_getattr=None):
+    lock = RLock()  # This is global for the class, can that be avoided?
+
+    def __getattr__(instance, item: str):
+        func = cached_properties.get(item)
+        if func is not None:
+            with lock:
+                try:
+                    # In case another thread has set it.
+                    return object.__getattribute__(instance, item)
+                except AttributeError:
+                    result = func(instance)
+                    object.__setattr__(instance, item, result)
+                    return result
+        elif original_getattr is not None:
+            return original_getattr(instance, item)
+        else:
+            raise AttributeError(item)
+
+    return __getattr__
+
+
 def _frozen_setattrs(self, name, value):
     """
     Attached to frozen classes as __setattr__.
@@ -858,9 +882,31 @@ def _create_slots_class(self):
         ):
             names += ("__weakref__",)
 
+        cached_properties = {
+            name: cached_property.func
+            for name, cached_property in cd.items()
+            if isinstance(cached_property, functools.cached_property)
+        }
+
+        if cached_properties:
+            # Add cached properties to names for slotting.
+            names += tuple(cached_properties.keys())
+
+            if "__annotations__" in cd:
+                for name, func in cached_properties.items():
+                    annotation = inspect.signature(func).return_annotation
+                    if annotation is not inspect.Parameter.empty:
+                        cd["__annotations__"][name] = annotation
+
+            cd["__getattr__"] = _make_cached_property_getattr(
+                cached_properties, cd.get("__getattr__")
+            )
+            del cd[name]
+
         # 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,
@@ -874,6 +920,7 @@ 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__
@@ -910,7 +957,6 @@ def _create_slots_class(self):
                 else:
                     if match:
                         set_closure_cell(cell, cls)
-
         return cls
 
     def add_repr(self, ns):

tests/test_slots.py

@@ -3,7 +3,7 @@
 """
 Unit tests for slots-related functionality.
 """
-
+import functools
 import pickle
 import sys
 import types
@@ -14,6 +14,7 @@
 import pytest
 
 import attr
+import attrs
 
 from attr._compat import PYPY, just_warn, make_set_closure_cell
 
@@ -747,6 +748,161 @@ def f(self):
     assert B(17).f == 289
 
 
+def test_slots_cached_property_allows_call():
+    """
+    cached_property in slotted class allows call.
+    """
+
+    @attr.s(slots=True)
+    class A:
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            return self.x
+
+    assert A(11).f == 11
+
+
+def test_slots_cached_property_class_does_not_have__dict__():
+    """
+    slotted class with cached property has no __dict__ attribute.
+    """
+
+    @attr.s(slots=True)
+    class A:
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            return self.x
+
+    assert set(A.__slots__) == {"x", "f", "__weakref__"}
+    assert "__dict__" not in dir(A)
+
+
+def test_slots_cached_property_infers_type():
+    """
+    Infers type of cached property.
+    """
+
+    @attrs.frozen(slots=True)
+    class A:
+        x: int
+
+        @functools.cached_property
+        def f(self) -> int:
+            return self.x
+
+    assert A.__annotations__ == {"x": int, "f": int}
+
+
+def test_slots_sub_class_avoids_duplicated_slots():
+    """
+    Duplicating the slots is a wast of memory.
+    """
+
+    @attr.s(slots=True)
+    class A:
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            return self.x
+
+    @attr.s(slots=True)
+    class B(A):
+        @functools.cached_property
+        def f(self):
+            return self.x * 2
+
+    assert B(1).f == 2
+    assert B.__slots__ == ()
+
+
+def test_slots_sub_class_with_actual_slot():
+    """
+    A sub-class can have an explicit attrs field that replaces a cached property.
+    """
+
+    @attr.s(slots=True)
+    class A:  # slots : (x, f)
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            return self.x
+
+    @attr.s(slots=True)
+    class B(A):
+        f: int = attr.ib()
+
+    assert B(1, 2).f == 2
+    assert B.__slots__ == ()
+
+
+def test_slots_cached_property_is_not_called_at_construction():
+    """
+    A cached property function should only be called at property access point.
+    """
+    call_count = 0
+
+    @attr.s(slots=True)
+    class A:
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            nonlocal call_count
+            call_count += 1
+            return self.x
+
+    A(1)
+    assert call_count == 0
+
+
+def test_slots_cached_property_repeat_call_only_once():
+    """
+    A cached property function should be called only once, on repeated attribute access.
+    """
+    call_count = 0
+
+    @attr.s(slots=True)
+    class A:
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            nonlocal call_count
+            call_count += 1
+            return self.x
+
+    obj = A(1)
+    obj.f
+    obj.f
+    assert call_count == 1
+
+
+def test_slots_cached_property_called_independent_across_instances():
+    """
+    A cached property value should be specific to the given instance.
+    """
+
+    @attr.s(slots=True)
+    class A:
+        x = attr.ib()
+
+        @functools.cached_property
+        def f(self):
+            return self.x
+
+    obj_1 = A(1)
+    obj_2 = A(2)
+
+    assert obj_1.f == 1
+    assert obj_2.f == 2
+
+
 @attr.s(slots=True)
 class A:
     x = attr.ib()