Add Turn Method Into Cached Property On Class Instance as a Python TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
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@@ -1090,6 +1090,7 @@ If you've learned something here, support my efforts writing daily TILs by
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 - [Store And Access Immutable Data In A Tuple](python/store-and-access-immutable-data-in-a-tuple.md)
 - [Test A Function With Pytest](python/test-a-function-with-pytest.md)
+- [Turn Method Into Cached Property On Class Instance](python/turn-method-into-cached-property-on-class-instance.md)
 - [Use pipx To Install End User Apps](python/use-pipx-to-install-end-user-apps.md)
 - [Use `__post_init__` For `dataclass` Validations](python/use-post-init-for-dataclass-validations.md)
 - [Use Verbose Flag To Get More Diff](python/use-verbose-flag-to-get-more-diff.md)

python/turn-method-into-cached-property-on-class-instance.md

@@ -0,0 +1,48 @@
+# Turn Method Into Cached Property On Class Instance
+
+I have a class that encapsulates a few things including a somewhat expensive
+data lookup from a file on disk. When this class is instantiated, it is
+short-lived and the data that gets pulled from the file on disk is considered
+fresh for the life of the instance.
+
+```python
+class CliContext:
+    def __init__(self, verbose: bool) -> None:
+        # ...
+        self.repo = JsonRepository()
+        # ...
+
+    def session_log(self) -> list[Session]:
+        return self.repo.load_session_log()
+```
+
+Because this method gets called from a couple places during a single lifecycle,
+this class would benefit from caching it via the [`@cached_property`
+decorator](https://docs.python.org/3/library/functools.html#functools.cached_property).
+
+```python
+from functools import cached_property
+
+class CliContext:
+    def __init__(self, verbose: bool) -> None:
+        # ...
+        self.repo = JsonRepository()
+        # ...
+
+    @cached_property
+    def session_log(self) -> list[Session]:
+        return self.repo.load_session_log()
+```
+
+Now `session_log` can be treated like a property instead of a method. That means
+when I want to load and access the session log, I can do `self.session_log` (no
+parentheses) like I would any other property. The first time I reference it, the
+method will run. Then that value will be cached and all subsequent references
+will use that cache.
+
+> Transform a method of a class into a property whose value is computed once and
+> then cached as a normal attribute for the life of the instance.
+
+Of course, anytime we use caching, we can create a footgun for ourselves. We
+have to be careful that our program doesn't evolve in such a way where the
+caching will create a subtle bug due to stale data.