Add a @cached_property implementation

This is a useful utility to abstract the caching property idiom.

It is in compat.py since eventually it will be replaced by
functools.cached_property.

Fixes #6131.

src/_pytest/compat.py

@@ -10,7 +10,11 @@ import sys
 from contextlib import contextmanager
 from inspect import Parameter
 from inspect import signature
+from typing import Callable
+from typing import Generic
+from typing import Optional
 from typing import overload
+from typing import TypeVar
 
 import attr
 import py
@@ -20,6 +24,13 @@ from _pytest._io.saferepr import saferepr
 from _pytest.outcomes import fail
 from _pytest.outcomes import TEST_OUTCOME
 
+if False:  # TYPE_CHECKING
+    from typing import Type  # noqa: F401 (used in type string)
+
+
+_T = TypeVar("_T")
+_S = TypeVar("_S")
+
 
 NOTSET = object()
 
@@ -374,3 +385,33 @@ if getattr(attr, "__version_info__", ()) >= (19, 2):
     ATTRS_EQ_FIELD = "eq"
 else:
     ATTRS_EQ_FIELD = "cmp"
+
+
+if sys.version_info >= (3, 8):
+    # TODO: Remove type ignore on next mypy update.
+    # https://github.com/python/typeshed/commit/add0b5e930a1db16560fde45a3b710eefc625709
+    from functools import cached_property  # type: ignore
+else:
+
+    class cached_property(Generic[_S, _T]):
+        __slots__ = ("func", "__doc__")
+
+        def __init__(self, func: Callable[[_S], _T]) -> None:
+            self.func = func
+            self.__doc__ = func.__doc__
+
+        @overload
+        def __get__(
+            self, instance: None, owner: Optional["Type[_S]"] = ...
+        ) -> "cached_property[_S, _T]":
+            raise NotImplementedError()
+
+        @overload  # noqa: F811
+        def __get__(self, instance: _S, owner: Optional["Type[_S]"] = ...) -> _T:
+            raise NotImplementedError()
+
+        def __get__(self, instance, owner=None):  # noqa: F811
+            if instance is None:
+                return self
+            value = instance.__dict__[self.func.__name__] = self.func(instance)
+            return value

src/_pytest/nodes.py

@@ -15,6 +15,7 @@ import _pytest._code
 from _pytest._code.code import ExceptionChainRepr
 from _pytest._code.code import ExceptionInfo
 from _pytest._code.code import ReprExceptionInfo
+from _pytest.compat import cached_property
 from _pytest.compat import getfslineno
 from _pytest.fixtures import FixtureDef
 from _pytest.fixtures import FixtureLookupError
@@ -448,17 +449,9 @@ class Item(Node):
     def reportinfo(self) -> Tuple[str, Optional[int], str]:
         return self.fspath, None, ""
 
-    @property
+    @cached_property
     def location(self) -> Tuple[str, Optional[int], str]:
-        try:
-            return self._location
-        except AttributeError:
         location = self.reportinfo()
         fspath = self.session._node_location_to_relpath(location[0])
         assert type(location[2]) is str
-            self._location = (
-                fspath,
-                location[1],
-                location[2],
-            )  # type: Tuple[str, Optional[int], str]
-            return self._location
+        return (fspath, location[1], location[2])

testing/test_compat.py

@@ -4,6 +4,7 @@ from functools import wraps
 
 import pytest
 from _pytest.compat import _PytestWrapper
+from _pytest.compat import cached_property
 from _pytest.compat import get_real_func
 from _pytest.compat import is_generator
 from _pytest.compat import safe_getattr
@@ -178,3 +179,23 @@ def test_safe_isclass():
             assert False, "Should be ignored"
 
     assert safe_isclass(CrappyClass()) is False
+
+
+def test_cached_property() -> None:
+    ncalls = 0
+
+    class Class:
+        @cached_property
+        def prop(self) -> int:
+            nonlocal ncalls
+            ncalls += 1
+            return ncalls
+
+    c1 = Class()
+    assert ncalls == 0
+    assert c1.prop == 1
+    assert c1.prop == 1
+    c2 = Class()
+    assert ncalls == 1
+    assert c2.prop == 2
+    assert c1.prop == 1