How to make arguments in python decorator configurable?

Question

I am using cachetools for some basic caching. Here is an example of how I am using it:

class Access:

  @cached(cache=TTLCache(maxsize=5, ttl=10))
  def get_some_value(input: str):
    # do some calls and return a value

The problem here is how do I make the maxsize and ttl configurable? I cannot do something like

class Access:
  def __init__(self, maxsize: int = 5, ttl: int = 10):
    self.maxsize = maxsize
    self.ttl = ttl

  @cached(cache=TTLCache(maxsize=self.maxsize, ttl=self.ttl))
  def get_some_value(input: str):
    # do some calls and return a value

I am looking for a way to inject those values if required and also have a default. Any helpful pointers? Also, get_some_value() need not be an instance method. I could just make it a class method or module level also if need be.

Answer 1

To deal with this, we can use the fact that Python decorators are simply functions that return other functions.

Suppose you have this eggs decorator:

def eggs(foo=10, bar=20):
    def wrapper_gen(func):
        def wrapper(*args):
            print(foo, bar)
            func(*args)
        return wrapper
    return wrapper_gen

And this Spam class:

class Spam:
    @eggs(foo=10, bar=20)
    def baz(self, input):
        print(input)

We can call the baz method as such:

Spam().baz("Hello, world!")

And this gives us

10 20
Hello, world!

Now, instead of directly decorating the function, we'll decorate in our __init__ method:

class Spam:
    def __init__(self, foo=10, bar=20):
        self.baz = eggs(foo=foo, bar=bar)(self._baz_func)

    def _baz_func(self, input):
        print(input)

And now:

Spam(foo=20, bar=30).baz("Hello, world!")

This outputs

20 30
Hello, world!

---

The reason this works is that this:

@foo
def bar():
    ...

is shorthand for this:

def bar():
    ...

bar = foo(bar)

Answer 2

This can't work using the regular decorator syntax, for two reasons: get_some_value belongs to the class, so it cannot have different behaviour for different instances since there's only one "copy" of it; and the decorator is executed at the time of the class declaration, not at the time of the instance creation, so there are no __init__ arguments yet.

However, you can get the result you want by applying the decorator explicitly in the __init__ method:

class Access:
    def __init__(self, maxsize: int = 5, ttl: int = 10):
        decorator = cached(cache=TTLCache(maxsize=maxsize, ttl=ttl))
        self.get_some_value = decorator(self.get_some_value)

    def get_some_value(self, input: str):
        ...