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