Make use of @cached_property

[blueyed]

@cached_property would be nice for this idiom. Maybe we should start using it (with a compat implementation).

Originally posted by @bluetech in #6129

[asottile]

fwiw, I factored out the cached-property """backport""" (technically it came first) package from pre-commit as it slowed import time by ~150ms (it imports asyncio unconditionally) -- worth considering when choosing a backport approach

[bluetech]

I implemented this, but could not find any other place besides the one in the original post where it might be useful. Just for that it does not seem worth the compat overhead, so my inclination is to shelve the idea.

patch

From e2c423d05e7e5fbdecc7334b7001b3ff90ba139e Mon Sep 17 00:00:00 2001
From: Ran Benita <ran@unusedvar.com>
Date: Wed, 6 Nov 2019 10:24:09 +0200
Subject: [PATCH] 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.
---
 src/_pytest/compat.py | 37 +++++++++++++++++++++++++++++++++++++
 src/_pytest/nodes.py  | 19 ++++++-------------
 2 files changed, 43 insertions(+), 13 deletions(-)

diff --git a/src/_pytest/compat.py b/src/_pytest/compat.py
index 83947d3eb..56229f020 100644
--- a/src/_pytest/compat.py
+++ b/src/_pytest/compat.py
@@ -10,6 +10,10 @@ 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 TypeVar
 from typing import overload
 
 import attr
@@ -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:
+
+
+_T = TypeVar("_T")
+_S = TypeVar("_S")
+
 
 NOTSET = object()
 
@@ -374,3 +385,29 @@ if getattr(attr, "__version_info__", ()) >= (19, 2):
     ATTRS_EQ_FIELD = "eq"
 else:
     ATTRS_EQ_FIELD = "cmp"
+
+
+# Replace with functools.cached_property once we only support
+# Python 3.8.
+class cached_property(Generic[_S, _T], property):
+    __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]":
+        pass
+
+    @overload  # noqa: F811
+    def __get__(self, instance: _S, owner: Optional["Type[_S]"] = ...) -> _T:
+        pass
+
+    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
diff --git a/src/_pytest/nodes.py b/src/_pytest/nodes.py
index d72eaeb0a..cdc39de71 100644
--- a/src/_pytest/nodes.py
+++ b/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
@@ -447,17 +448,9 @@ 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
+        location = self.reportinfo()
+        fspath = self.session._node_location_to_relpath(location[0])
+        assert type(location[2]) is str
+        return (fspath, location[1], location[2])
-- 
2.23.0

[blueyed]

@bluetech
Thanks!
I am pretty sure there are other places (now already, or in the future), so I'm taking this into my fork already (blueyed#84).
I think having it available is useful in general, and suggest to create a PR for pytest itself also.

[RonnyPfannschmidt]

@blueyed what exactly is the intent of your personal fork - i found it and how its maintained confusing as i lack context

[blueyed]

@RonnyPfannschmidt

what exactly is the intent of your personal fork

This is mainly for getting things done, and having things here that I like better.

I have started with https://github.com/blueyed/pytest-enhancements, but figured
that it is more sane to have a personal fork than a plugin that has to
monkeypatch pytest anyway.

It also allows for having CI etc (in contrast to having this only in a local
integration branch).