Proper way of using properties with try and except

Question

I have classes that have attributes set with @property decorator. They function as getter and setter using try and except clauses inside them. If attribute is not s

Answer 1

So long as you are using at least Python 3.2, use the functools.lru_cache() decorator.

import functools
class SubClass(TopClass):

    @property
    @functools.lru_cache()
    def thing(self):
        thing = get_some_thing_from_db('thing')
        if not thing:
            raise AttributeError()
        return TheThing(thing)

A quick runnable example:

>>> import functools
>>> class C:
    @property
    @functools.lru_cache()
    def foo(self):
        print("Called foo")
        return 42


>>> c = C()
>>> c.foo
Called foo
42
>>> c.foo
42

If you have a lot of these you can combine the decorators:

>>> def lazy_property(f):
    return property(functools.lru_cache()(f))

>>> class C:
    @lazy_property
    def foo(self):
        print("Called foo")
        return 42


>>> c = C()
>>> c.foo
Called foo
42
>>> c.foo
42

If you are still on an older version of Python there's a fully featured backport of lru_cache on ActiveState although as in this case you're not passing any parameters when you call it you could probably replace it with something much simpler.

@YAmikep asks how to access the cache_info() method of lru_cache. It's a little bit messy, but you can still access it through the property object:

>>> C.foo.fget.cache_info()
CacheInfo(hits=0, misses=1, maxsize=128, currsize=1)