Caching class attributes in Python

The most simple way of doing this would probably be to just write a method (instead of using an attribute) that wraps around the attribute (getter method). On the first call, this methods calculates, saves and returns the value; later it just returns the saved value.

The usual way would be to make the attribute a property and store the value the first time it is calculated

import time

class Foo(object):
    def __init__(self):
        self._bar = None

    @property
    def bar(self):
        if self._bar is None:
            print "starting long calculation"
            time.sleep(5)
            self._bar = 2*2
            print "finished long caclulation"
        return self._bar

foo=Foo()
print "Accessing foo.bar"
print foo.bar
print "Accessing foo.bar"
print foo.bar

You could try looking into memoization. The way it works is that if you pass in a function the same arguments, it will return the cached result. You can find more information on implementing it in python here.

Also, depending on how your code is set up (you say that it is not needed by all instances) you could try to use some sort of flyweight pattern, or lazy-loading.

class MemoizeTest:

      _cache = {}
      def __init__(self, a):
          if a in MemoizeTest._cache:
              self.a = MemoizeTest._cache[a]
          else:
              self.a = a**5000
              MemoizeTest._cache.update({a:self.a})

I used to do this how gnibbler suggested, but I eventually got tired of the little housekeeping steps.

So I built my own descriptor:

class cached_property(object):
    """
    Descriptor (non-data) for building an attribute on-demand on first use.
    """
    def __init__(self, factory):
        """
        <factory> is called such: factory(instance) to build the attribute.
        """
        self._attr_name = factory.__name__
        self._factory = factory

    def __get__(self, instance, owner):
        # Build the attribute.
        attr = self._factory(instance)

        # Cache the value; hide ourselves.
        setattr(instance, self._attr_name, attr)

        return attr

Here’s how you’d use it:

class Spam(object):

    @cached_property
    def eggs(self):
        print 'long calculation here'
        return 6*2

s = Spam()
s.eggs      # Calculates the value.
s.eggs      # Uses cached value.

With Python 2, but not Python 3, here’s what I do. This is about as efficient as you can get:

class X:
    @property
    def foo(self):
        r = 33
        self.foo = r
        return r

Explanation: Basically, I’m just overloading a property method with the computed value. So after the first time you access the property (for that instance), foo ceases to be a property and becomes an instance attribute. The advantage of this approach is that a cache hit is as cheap as possible because self.__dict__ is being used as the cache, and there is no instance overhead if the property is not used.

This approach doesn’t work with Python 3.

Python 3.8 includes the functools.cached_property decorator.

Transform a method of a class into a property whose value is computed
once and then cached as a normal attribute for the life of the
instance. Similar to property(), with the addition of caching. Useful
for expensive computed properties of instances that are otherwise
effectively immutable.

This example is straight from the docs:

from functools import cached_property

class DataSet:
    def __init__(self, sequence_of_numbers):
        self._data = sequence_of_numbers

    @cached_property
    def stdev(self):
        return statistics.stdev(self._data)

    @cached_property
    def variance(self):
        return statistics.variance(self._data)

The limitation being that the object with the property to be cached must have a __dict__ attribute that is a mutable mapping, ruling out classes with __slots__ unless __dict__ is defined in __slots__.

The dickens package (not mine) offers cachedproperty, classproperty and cachedclassproperty decorators.

To cache a class property:

from descriptors import cachedclassproperty

class MyClass:
    @cachedclassproperty
    def approx_pi(cls):
        return 22 / 7

Most if not all current answers are about caching instance attributes. To cache class attributes, you can simply use a dictionary. This ensures the attributes are calculated once per class, instead of once per instance.

mapping = {}

class A:
    def __init__(self):
        if self.__class__.__name__ not in mapping:
            print('Expansive calculation')
            mapping[self.__class__.__name__] = self.__class__.__name__
        self.cached = mapping[self.__class__.__name__]

To illustrate,

foo = A()
bar = A()
print(foo.cached, bar.cached)

gives

Expansive calculation
A A

As mentioned, functools.cached_property will work for cached instance attributes. For cached class attributes:

from functools import cache

class MyClass:
    @classmethod
    @property
    @cache
    def foo(cls):
        print('expensive calculation')
        return 42

>>> MyClass.foo
expensive calculation
42
>>> MyClass.foo
42

And if you want a reusable decorator:

def cached_class_attr(f):
    return classmethod(property(cache(f)))

class MyClass:
    @cached_class_attr
    def foo(cls):
        ...

3.9 >= python < 3.11