Is accessing class variables via an instance documented?

Question

In Python, class variables can be accessed via that class instance:

>>> class A(object):
...     x = 4
...
>>> a = A()
>>> a.x
4

It’s easy to show that a.x is really resolved to A.x, not copied to an instance during construction:

>>> A.x = 5
>>> a.x
5

Despite the fact that this behavior is well known and widely used, I couldn’t find any definitive documentation covering it. The closest I could find in Python docs was the section on classes:

class MyClass:
    """A simple example class"""
    i = 12345
    def f(self):
        return 'hello world'
[snip]

… By definition, all attributes of a class that are function objects define corresponding methods of its instances. So in our example, x.f is a valid method reference, since MyClass.f is a function, but x.i is not, since MyClass.i is not. …

However, this part talks specifically about methods so it’s probably not relevant to the general case.

My question is, is this documented? Can I rely on this behavior?

Asked By: SnakE

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Source

Answer 1

Refs the Classes and Class instances parts in the Python data model documentation

A class has a namespace implemented by a dictionary object. Class
attribute references are translated to lookups in this dictionary,
e.g., C.x is translated to C.__dict__["x"] (although for new-style classes in particular there are a number of hooks which allow for other means of locating attributes).

…

A class instance is created by calling a class object (see above). A
class instance has a namespace implemented as a dictionary which is
the first place in which attribute references are searched. When an
attribute is not found there, and the instance’s class has an
attribute by that name, the search continues with the class
attributes.

Generally, this usage is fine, except the special cases mentioned as "for new-style classes in particular there are a number of hooks which allow for other means of locating attributes".

Answered By: okm

Answer 2

Not only can you rely on this behavior, you constantly do.

Think about methods. A method is merely a function that has been made a class attribute. You then look it up on the instance.

>>> def foo(self, x):
...     print "foo:", self, x
... 
>>> class C(object):
...     method = foo # What a weird way to write this! But perhaps illustrative?
... 
>>> C().method("hello")
foo: <__main__.C object at 0xadad50> hello

In the case of objects like functions, this isn’t a plain lookup, but some magic occurs to pass self automatically. You may have used other objects that are meant to be stored as class attributes and looked up on the instance; properties are an example (check out the property builtin if you’re not familiar with it.)

As okm notes, the way this works is described in the data model reference (including information about and links to more information about the magic that makes methods and properties work). The Data Model page is by far the most useful part of the Language Reference; it also includes among other things documentation about almost all the __foo__ methods and names.

Answered By: Mike Graham

Is accessing class variables via an instance documented?

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