cache-results

A Django decorator that caches a function's results by its arguments, in any Django cache, such as Redis.

from cache_results import cache_results


@cache_results(timeout=600)
def get_book(book_id: int) -> dict:
    return Book.objects.values().get(pk=book_id)


get_book(42)             # computed and cached
get_book(42)             # served from the cache
get_book.invalidate(42)  # clears that entry

Documentation: https://docs.cache-results.smarter.sh/

cache-results grew out of the caching layer of Smarter, where it caches database reads and third-party api calls across many application servers.

Features

• Safe keys. The key is the function's module and qualified name, plus a hash of its arguments bound to its signature, so f(1), f(a=1) and f(1, b=2) (when b defaults to 2) share one entry. Arguments that can't be part of a safe key, such as model instances, run the function uncached and log an error, rather than be keyed by str().
• None is cached like any other result.
• **.invalidate(\*args, **kwargs)** on every decorated function clears the entry of those arguments.
• async functions are cached without blocking the event loop.
• Classmethods are cached per class. Instance methods are refused, because self would be part of the key.
• No stampede within a process. Threads that miss the same key at once wait for the first one to compute it.
• Safe during startup. A function called before Django's apps are ready runs uncached, rather than initialize the cache framework too early.
• lazy_cache, Django's cache imported only once Django is ready, for raw get() and set().
• Configurable through one Django setting, CACHE_RESULTS.
• Depends only on Django and asgiref.

Installation

Install it from PyPI:

pip install cache-results

It needs Python 3.10 or newer and Django 4.2 or newer, and works with any Django cache backend. With a shared cache such as Redis, cached results survive restarts and are shared by every application server.

Arguments

The arguments are the cache key, so they must be plain values: str, int, float, bool, None, and lists, tuples, dicts and sets of them, plus datetimes, Decimals, UUIDs, classes and dataclasses. Pass a model's id, not the model:

@cache_results()
def get_profile(user_id: int) -> Profile:
    return Profile.objects.select_related("user").get(user_id=user_id)


get_profile(request.user.pk)

To allow another type, subclass CacheKeyEncoder and name your subclass in the KEY_ENCODER setting:

from cache_results import CacheKeyEncoder


class MyCacheKeyEncoder(CacheKeyEncoder):
    def default(self, o):
        if isinstance(o, Money):
            return {"__money__": [str(o.amount), o.currency]}
        return super().default(o)

Never pass a secret, such as an api key, as an argument: arguments are part of the key, and verbose logging writes them to the log. Pass the id of the record that holds it, and read the secret inside the function.

Functions defined inside an inherited classmethod

A qualified name records where a function is defined, not who calls it. A function defined inside a classmethod that subclasses inherit has the same qualified name for every subclass, so subclasses would share its entries. Give it the class name as a parameter:

class Resource(models.Model):
    class Meta:
        abstract = True

    @classmethod
    def get_cached(cls, pk: int):
        @cache_results()
        def _get(pk: int, class_name: str = cls.__name__):
            return cls.objects.get(pk=pk)

        return _get(pk)

Settings

Every entry of the CACHE_RESULTS setting is optional:

CACHE_RESULTS = {
    # the CACHES alias that holds cached results.
    "CACHE_ALIAS": "default",
    # the default timeout, in seconds, of a decorated function that names none.
    # Unset, it is the cache alias's own TIMEOUT.
    "TIMEOUT": 3600,
    # text that starts every key. Changing it, or VERSION, abandons every cached
    # result at once, without touching anything else in the same cache.
    "KEY_PREFIX": "",
    "VERSION": None,
    # a subclass of cache_results.CacheKeyEncoder, to allow more argument types.
    "KEY_ENCODER": "cache_results.keys.CacheKeyEncoder",
    # zero-argument callables that return whether to log cache hits, misses and
    # invalidations, and whether those logs include the arguments.
    "LOGGING_ENABLED": None,
    "VERBOSE_LOGGING": None,
    # a callable(text, event) that formats the start of each log message, where
    # event is "hit", "miss", "invalidate" or "".
    "LOG_PREFIX": None,
    # with LOG_PREFIX = "cache_results.colors.log_prefix", which colors log
    # messages by event: a dict from event to ANSI code.
    "LOG_COLORS": None,
}

Callables and classes are given as dotted paths or as the objects themselves. Settings are read on every call, so override_settings works in tests.

lazy_cache

For raw get() and set(), use lazy_cache in place of django.core.cache.cache. It has the same api, and imports Django's cache only on first use, once Django's apps are ready: importing it while Django is starting can leave it on a local-memory backend that persists nothing.

from cache_results import lazy_cache as cache

cache.set("my_key", "my_value", timeout=300)
cache.get("my_key")  # "my_value"

On first use it checks the cache, and warns when the default cache is a local-memory or dummy backend.

Decorator options

@cache_results(timeout=None, cache_key=None, logging_enabled=False)

• timeout: seconds. None uses the TIMEOUT setting, and 0 caches nothing.
• cache_key: a fixed key that every call shares, whatever its arguments. Use it only for functions whose result doesn't depend on them.
• logging_enabled: log this function's hits, misses and invalidations even when the LOGGING_ENABLED setting is off. Logs name the function and the type of its result, never the result.

Development

make            # list every command
make init       # create venv/ from requirements/local.txt, install the package and the git hooks
make test       # run the tests against the local-memory cache
make test-redis # run the tests against Redis
make coverage   # run the tests with a coverage report
make lint       # run the pre-commit hooks on every file
make typecheck  # check the type annotations with mypy
make sphinx-build   # build the Sphinx docs into docs/build/html
make sphinx-publish # publish them to docs.cache-results.smarter.sh
make build      # build the sdist and wheel, and check them
make python-requirements  # recompile the pinned requirements/*.txt

make test-redis uses redis://localhost:6379/15 unless you set REDIS_URL, and clears that database. Running pytest yourself, set CACHE_RESULTS_TEST_REDIS_URL instead.

CI runs the hooks, the tests on every supported pair of Python and Django versions and against Redis, and a build, on every pull request, and on every push to main, alpha and beta as the first step of the Release workflow. Coverage is reported to Codecov. When CI passes, the Release workflow releases with semantic-release, from the conventional commit messages, and publishes to PyPI from main (or to TestPyPI from alpha and beta). make release-dry-run shows the next version.

Contributing

Bug reports, questions and pull requests are welcome. Please open an issue before starting on a larger change, so that we can agree on its design first.

1. Fork the repository, and branch from main.
2. Run make init. It also installs the pre-commit hooks, which are the lint standard (black, flake8, isort, pyupgrade, codespell, bandit, Prettier) and check each commit message.
3. Make your change with its tests. make coverage must stay at 95% or more, and both make test and make test-redis must pass. CI checks all of this on your pull request.
4. Write conventional commits, e.g. fix(keys): sort frozensets of tuples. Releases are versioned from them: fix makes a patch release, feat a minor one.
5. Open a pull request that says what changed and why, and links its issue.

A few rules keep @cache_results() safe and readable:

• The package depends only on Django and asgiref. Project-specific behavior belongs in a project's own hooks, named by the CACHE_RESULTS setting.
• Arguments are never keyed by str(), and results and secrets are never logged.
• A change to the key format, or to the value stored for None, makes every cached entry unreadable, so it needs a major release.

Governance of the Smarter projects is described in smarter-sh/community. Everyone who takes part is expected to follow the code of conduct.

License

MIT