A flexible decorator

A flexible decorator#

The decorator factory cache_factory is inspired by the decorator lru_cache from the module functools. But lru_cache is an interesting decorator, because it can be used in three key different ways, from a syntactical point of view:

@lru_cache        # 1
@lru_cache()      # 2
@lru_cache(1000)  # 3

  1. You can use the decorator directly, without parametrising it.

  2. You can call the decorator with no arguments.

  3. You can customise the decorator to specify the maximum size.

From the point of view of the behaviour of the cache, 1. and 2. are the same, and create caches with no maximum size. But from the point of view of the syntax and the things that Python does behind the scenes, the three alternatives are fairly different. If you unroll the syntactic sugar of the at sign @, this is what you get:

fn = lru_cache(fn)        # 1
fn = lru_cache()(fn)      # 2
fn = lru_cache(1000)(fn)  # 3

How do you write a decorator that can behave both like a decorator factory (as in 1.) and like a decorator (as in 2. and 3.)?

First, you’ll see how to create a decorator that works with optional arguments, as in 2. and 3. Then, you’ll see how to conjugate that with using the decorator directly, as in 1.