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How do you modify the decorator factory cache_factory so that it can be used directly as a decorator, with the syntax @cache_factory? Suppose that this is already possible. In that case, using the decorator looks like this:

@cache_factory
def fn(*args):
    ...

Again, because the at sign @ is just syntactic sugar, the code above is the same as

def fn(*args):
    ...

fn = cache_factory(fn)

So, it looks like the function fn is passed into cache_factory as the argument maxsize! Therefore, for everything to work without any issues, the decorator factory cache_factory must be modified:

• if the argument maxsize is an integer, then cache_factory was used as a decorator factory, as it was originally intended; and

• if the argument maxsize is actually a function, then cache_factory was used directly as a decorator and the cache factory must produce a cache with the default value for maxsize; in a way, you have to skip a step.

Inside cache_factory, you can use the built-in callable to check whether your argument maxsize is callable (which means you accepted a function) or not, and then you can act accordingly. In terms of code, it looks like this:

def cache_factory(maxsize_or_fn=None):
    def cache(f):
        ...

    if callable(maxsize_or_fn):
        maxsize = None
        return cache(f)
    else:
        return cache

If the argument is a function, then the function you want to decorate is in the variable maxsize_or_fn and the value for maxsize should be restored to the default value of None, so you assign None to maxsize and then return the decorator applied to the function you wish to decorate.

If the argument is not a function, then cache_factory was used as a regular cache factory and you’re supposed to return the decorator.

Take a moment to get comfortable with this, as it can be pretty mind-bending the first time you see it!