I would like to know what is the python way of initializing a class member but only when accessing it, if accessed. I tried the code below and it is working but is there somethi
Another approach to make the code cleaner is to write a wrapper function that does the desired logic:
def memoize(f):
def wrapped(*args, **kwargs):
if hasattr(wrapped, '_cached_val'):
return wrapped._cached_val
result = f(*args, **kwargs)
wrapped._cached_val = result
return result
return wrapped
You can use it as follows:
@memoize
def expensive_function():
print "Computing expensive function..."
import time
time.sleep(1)
return 400
print expensive_function()
print expensive_function()
print expensive_function()
Which outputs:
Computing expensive function...
400
400
400
Now your classmethod would look as follows, for example:
class MyClass(object):
@classmethod
@memoize
def retrieve_data(cls):
print "Computing data"
import time
time.sleep(1) #costly DB call
my_data = 40
return my_data
print MyClass.retrieve_data()
print MyClass.retrieve_data()
print MyClass.retrieve_data()
Output:
Computing data
40
40
40
Note that this will cache just one value for any set of arguments to the function, so if you want to compute different values depending on input values, you'll have to make memoize
a bit more complicated.
You could use a @property on the metaclass instead:
class MyMetaClass(type):
@property
def my_data(cls):
if getattr(cls, '_MY_DATA', None) is None:
my_data = ... # costly database call
cls._MY_DATA = my_data
return cls._MY_DATA
class MyClass(metaclass=MyMetaClass):
# ...
This makes my_data
an attribute on the class, so the expensive database call is postponed until you try to access MyClass.my_data
. The result of the database call is cached by storing it in MyClass._MY_DATA
, the call is only made once for the class.
For Python 2, use class MyClass(object):
and add a __metaclass__ = MyMetaClass attribute in the class definition body to attach the metaclass.
Demo:
>>> class MyMetaClass(type):
... @property
... def my_data(cls):
... if getattr(cls, '_MY_DATA', None) is None:
... print("costly database call executing")
... my_data = 'bar'
... cls._MY_DATA = my_data
... return cls._MY_DATA
...
>>> class MyClass(metaclass=MyMetaClass):
... pass
...
>>> MyClass.my_data
costly database call executing
'bar'
>>> MyClass.my_data
'bar'
This works because a data descriptor like property
is looked up on the parent type of an object; for classes that's type
, and type
can be extended by using metaclasses.
Consider the pip-installable Dickens package which is available for Python 3.5+. It has a descriptors
package which provides the relevant cachedproperty
and cachedclassproperty
decorators, the usage of which is shown in the example below. It seems to work as expected.
from descriptors import cachedproperty, classproperty, cachedclassproperty
class MyClass:
FOO = 'A'
def __init__(self):
self.bar = 'B'
@cachedproperty
def my_cached_instance_attr(self):
print('Initializing and caching attribute, once per class instance.')
return self.bar * 2
@cachedclassproperty
def my_cached_class_attr(cls):
print('Initializing and caching attribute, once per class.')
return cls.FOO * 3
@classproperty
def my_class_property(cls):
print('Calculating attribute without caching.')
return cls.FOO + 'C'
Ring
gives lru_cache
-like interface but working with any kind of descriptor supports: https://ring-cache.readthedocs.io/en/latest/quickstart.html#method-classmethod-staticmethod
class Page(object):
(...)
@ring.lru()
@classmethod
def class_content(cls):
return cls.base_content
@ring.lru()
@staticmethod
def example_dot_com():
return requests.get('http://example.com').content
See the link for more details.
This answer is for a typical instance attribute/method only, not for a class attribute/classmethod
, or staticmethod
.
For Python 3.8+, how about using the cached_property decorator? It memoizes.
from functools import cached_property
class MyClass:
@cached_property
def my_lazy_attr(self):
print("Initializing and caching attribute, once per class instance.")
return 7**7**8
For Python 3.2+, how about using both property and lru_cache decorators? The latter memoizes.
from functools import lru_cache
class MyClass:
@property
@lru_cache()
def my_lazy_attr(self):
print("Initializing and caching attribute, once per class instance.")
return 7**7**8
Credit: answer by Maxime R.