Question

Question

Is it possible to instantiate an object, set an attribute of that object equal to a class method but delay the calling of that method while enabling access to that attribute (obj.name) without having to call it as a method (obj.name())

Background

I have a class that instantiates an object. Part of that instantiation is setting an attribute equal to a database object, which requires a lookup. This lookup, when instantiating many objects (several hundred), can be slow.

I would like to somehow delay that lookup until that information is needed. However, I don’t want to have to call a method on the object to do that lookup, I would like to simply access the attribute (object.attribute)

Simple Example / What I’ve Tried So Far

class Article(object):

    def __init__(self, id, author):
        self.id = id
        # Note the lack of () after lookup_author below
        self.author = self.lookup_author

        # Temporary holding place for author data
        self.__author = author

    def lookup_author(self):
        # A lookup that would be nice to delay / run as needed
        # Would be something like Author.objects.get(author=self.__author)
        # but set to something simple for this example
        return '<Author: John Doe>'

article1 = Article(1, 'John Doe')

# Returns the bound method
# E.g. <bound method Article.lookup_author of <__main__.Article object at 0x100498950>>
print article1.author

# Calls the method properly, however, you have to use the method calling
# notation of .state() versus .state which is more natural and expected
# for attributes
# Returns <Author: John Doe>
print article1.author()

Answer 1

Using properties, you can have article1.author actually call self.lookup_author and return it.

output:

John Doe
<Author: John Doe>

bob
<Author: bob>

code:

class Article(object):
    def __init__(self, id, author):
        self.id = id
        self.__author = None

    def lookup_author(self):
        return "John Doe"


    def __str__(self):
        return "<Author: {}>".format(self.author)

    @property
    def author(self):
        if self.__author is None:
            self.__author = self.lookup_author()
        return self.__author

    @author.setter
    def author(self,name):
        self.__author = name

article1 = Article(1, 'John Doe')
print "\n", article1.author
print article1

article1.author = 'bob'
print "\n", article1.author
print article1

For some reason, if needed, __author doesn’t have to even exist until the getter is used. You can do that using exceptions.