Python class member lazy initialization

Question

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

Answer 1

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.