Differences between data attributes and method attributes

Question

What is a method attribute, and a data attribute? What the difference between them and what they have in common?

I was reading python 2.7.9 (https://docs.python.org/2/tutorial/classes.html#random-remarks) and suddenly both became hard to understand. I'll appreciate some light over it.

Answer 1

An attribute is a variable that is looked up on another object using dot syntax: obj.attribute. The way Python is designed, attribute lookups can do a variety of things, and that variety can sometimes lead to bugs if you don't really understand what is happening (this is what the documentation you linked to warns about).

The most basic issue is that an attribute lookup can find either a value stored in the object's instance dictionary, or it can find something from the object's class (or a base class, if there's inheritance going on). Methods are functions stored in the class, but you usually use them by looking them up on an instance (which "binds" the method, inserting the object as the first arguemnt when the method is called).

The exact sequence of what is checked when is a bit complicated (I described the full process in an answer to another question), but at the most basic level, instance attributes usually take precedence over class attribute.

If an instance attribute and a class attribute with the same name both exist, usually only the instance attribute will be accessible. This can be very confusing if it is unintended.

Consider the following code:

class Foo(object):
    def __init__(self, lst):
        self.lst = lst

    def sum(self):
        self.sum = sum(self.lst)
        return self.sum

f = Foo([1,2,3])

print(f.sum())
print(f.sum())

At the bottom of this code, we make two identical calls. The first works just fine, but the second will raise an exception.

This is because the first time we look up f.sum we find a method in the Foo class. We can call the method with no problems. The trouble comes from the fact that the sum method assigns the result of its calculation (the sum of the elements in self.lst) to an instance attribute also named sum. This hides the sum method from view.

When second f.sum() call looks up f.sum, it finds the instance attribute, containing the integer 6, rather than the expected method. An integer is not callable, so we get an exception.

The solution, of course, is not to use the same name for the method and attribute. The code above is a pretty trivial example. The bugs caused by this sort of thing in more complex code can be much more difficult to figure out.

If you're writing code that adds attributes to objects you don't know much about, you should be careful to avoid common names. If you're writing a mixin class, consider using two leading underscores in the attribute names to trigger Python's name mangling, which is designed for exactly this sort of situation.

Answer 2

An attribute is any thing for the lack of a better word that is bound to an object, for example:

class Dog:
    def __init__(self):
        self.name = "Rufus"

    def bark(self):
        print "Woof Woof!"

In this case the data attribute is the name, which is simply a value that is bound to the instance of the Dog. As for a method attribute, one answer would be the bark method, as it's not so much a value as it is an action. It's just as it is in English. A data attribute is exactly as it sounds; it's data, it is simply a property. A method is a procedure, an action, and this is exactly what a method attribute is.