cardinal_pythonlib.django.function_cache

Original code copyright (C) 2009-2022 Rudolf Cardinal (rudolf@pobox.com).

This file is part of cardinal_pythonlib.

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Cache the results of function calls using Django.

Based on https://github.com/rchrd2/django-cache-decorator but fixed for Python 3 / Django 1.10.

cardinal_pythonlib.django.function_cache.django_cache_function(timeout: int = 300, cache_key: str = '', debug_cache: bool = False)[source]

Decorator to add caching to a function in Django. Uses the Django default cache.

Parameters:

  • timeout – timeout in seconds; use None for “never expire”, as 0 means “do not cache”.

  • cache_key – optional cache key to use (if falsy, we’ll invent one)

  • debug_cache – show hits/misses?

cardinal_pythonlib.django.function_cache.get_call_signature(fn: Callable[[...], Any], args: Tuple[Any, ...], kwargs: Dict[str, Any], debug_cache: bool = False) → str[source]

Takes a function and its args/kwargs, and produces a string description of the function call (the call signature) suitable for use indirectly as a cache key. The string is a JSON representation. See make_cache_key for a more suitable actual cache key.

cardinal_pythonlib.django.function_cache.make_cache_key(call_signature: str, debug_cache: bool = False) → str[source]

Takes a function and its args/kwargs, and produces a string description of the function call (the call signature) suitable for use as a cache key. The string is an MD5 hash of the JSON-encoded call signature. The logic behind these decisions is as follows:

  • We have a bunch of components of arbitrary type, and we need to get a unique string out.

  • We shouldn’t use str(), because that is often poorly specified; e.g. is 'a.b.c' a TableId, or is it a ColumnId with no 'db' field?

  • We could use repr(): sometimes that gives us helpful things that could in principle be passed to eval(), in which case repr() would be fine, but sometimes it doesn’t, and gives unhelpful things like '<__main__.Thing object at 0x7ff3093ebda0>'.

  • However, if something encodes to JSON, that representation should be reversible and thus contain the right sort of information.

  • Note also that bound methods will come with a self argument, for which the address may be very relevant…

  • Let’s go with repr(). Users of the cache decorator should not pass objects whose repr() includes the memory address of the object unless they want those objects to be treated as distinct.

  • Ah, no. The cache itself will pickle and unpickle things, and this will change memory addresses of objects. So we can’t store a reference to an object using repr() and using cache.add()/pickle() and hope they’ll come out the same.

  • Use the JSON after all.

  • And do it in get_call_signature(), not here.

  • That means that any class we wish to decorate WITHOUT specifying a cache key manually must support JSON.