Pythonic way to only do work first time a variable is called

Question

Hello,

my Python class has some variables that require work to calculate the first time they are called. Subsequent calls should just return the precomputed value.

I don't want to waste time doing this work unless they are actually needed by the user. So is there a clean Pythonic way to implement this use case?

My initial thought was to use property() to call a function the first time and then override the variable:

class myclass(object):
    def get_age(self):
        self.age = 21 # raise an AttributeError here
        return self.age

    age = property(get_age)

Thanks

Answer 1

class myclass(object):
    def __init__(self):
        self.__age=None
    @property
    def age(self):
        if self.__age is None:
            self.__age=21  #This can be a long computation
        return self.__age

Alex mentioned you can use __getattr__, this is how it works

class myclass(object):
    def __getattr__(self, attr):
        if attr=="age":
            self.age=21   #This can be a long computation
        return super(myclass, self).__getattribute__(attr)

__getattr__() is invoked when the attribute doesn't exist on the object, ie. the first time you try to access age. Every time after, age exists so __getattr__ doesn't get called

Answer 2

Yes you can use properties, though lazy evaluation is also often accomplished using descriptors, see e.g:

http://blog.pythonisito.com/2008/08/lazy-descriptors.html

Answer 3

property, as you've seen, will not let you override it. You need to use a slightly different approach, such as:

class myclass(object):

    @property
    def age(self):
      if not hasattr(self, '_age'):
        self._age = self._big_long_computation()
      return self._age

There are other approaches, such as __getattr__ or a custom descriptor class, but this one is simpler!-)

Answer 4

Here is decorator from Python Cookbook for this problem:

class CachedAttribute(object):
    ''' Computes attribute value and caches it in the instance. '''
    def __init__(self, method, name=None):
        # record the unbound-method and the name
        self.method = method
        self.name = name or method.__name__
    def __get__(self, inst, cls):
        if inst is None:
            # instance attribute accessed on class, return self
            return self
        # compute, cache and return the instance's attribute value
        result = self.method(inst)
        setattr(inst, self.name, result)
        return result