Calculate attribute if it doesn't exist

Question

I am trying to access an attribute that shouldn\'t be created in the __init__ method of my class but can be calculated by calling another method. I am trying to

Answer 1

Pyramid (a Web framework) comes with a reify decorator that is similar to property (shown by Austin Hastings) but it works a little differently: the function is only executed once, and after that, the value returned by the function is always used. It essentially does what Austin's code does, but without having to use a separate attribute: it's a generalization of that pattern.

You probably don't want to use a whole Web framework just for this one decorator, so here is an equivalent one I wrote:

import functools

class Descriptor(object):
    def __init__(self, func):
        self.func = func
    def __get__(self, inst, type=None):
        val = self.func(inst)
        setattr(inst, self.func.__name__, val)
        return val

def reify(func):
    return functools.wraps(func)(Descriptor(func))

Usage:

class ReifyDemo:
    @reify
    def total(self):
        """Compute or return the total attribute."""
        print("calculated total")
        return 2 + 2    # some complicated calculation here

r = ReifyDemo()
print(r.total)     # prints 'calculated total 4' because the function was called
print(r.total)     # prints just '4` because the function did not need to be called

Answer 2

You want to use the @property decorator. Create a method, that will be accessed like a normal attribute, that does lazy computation:

class SampleObject:

    def __init__(self):
        # ...
        self._total = None

    @property
    def total(self):
        """Compute or return the _total attribute."""
        if self._total is None:
            self.compute_total()

        return self._total