Why accessing to class variable from within the cl

问题:

Possible Duplicate:
Python ‘self’ explained

I'm learning Python and I have a question, more theoretical than practical, regarding access class variables from method of this class.

For example we have:

class ExampleClass:
    x = 123
    def example_method(self):
        print(self.x)

Why is necessarily to write exactly self.x, not just x? x belongs to namespace of the class, and method using it belongs to it too. What am I missing? What a rationale stands behind such style?

In C++ you can write:

class ExampleClass {
public:
    int x;
    void example_method()
    {
        x = 123;
        cout << x;
    };
};

And it will work!

回答1:

From The History of Python: Adding Support for User-defined Classes:

Instead, I decided to give up on the idea of implicit references to instance variables. Languages like C++ let you write this->foo to explicitly reference the instance variable foo (in case there’s a separate local variable foo). Thus, I decided to make such explicit references the only way to reference instance variables. In addition, I decided that rather than making the current object ("this") a special keyword, I would simply make "this" (or its equivalent) the first named argument to a method. Instance variables would just always be referenced as attributes of that argument.

With explicit references, there is no need to have a special syntax for method definitions nor do you have to worry about complicated semantics concerning variable lookup. Instead, one simply defines a function whose first argument corresponds to the instance, which by convention is named "self." For example:

def spam(self,y):
    print self.x, y

This approach resembles something I had seen in Modula-3, which had already provided me with the syntax for import and exception handling. Modula-3 doesn’t have classes, but it lets you create record types containing fully typed function pointer members that are initialized by default to functions defined nearby, and adds syntactic sugar so that if x is such a record variable, and m is a function pointer member of that record, initialized to function f, then calling x.m(args) is equivalent to calling f(x, args). This matches the typical implementation of objects and methods, and makes it possible to equate instance variables with attributes of the first argument.

So, stated by the BDFL himself, the only real reason he decided to use explicit self over implicit self is that:

• it is explicit
• it is easier to implement, since the lookup must be done at runtime(and not at compile time like other languages) and having implicit self could have increased the complexity(and thus cost) of the lookups.

Edit: There is also an answer in the Python FAQ.

回答2:

It seems to be related to module vs. class scope handling, in Python:

COLOR = 'blue'

class TellColor(object):
    COLOR = 'red'

    def tell(self):
        print self.COLOR   # references class variable
        print COLOR        # references module variable

a = TellColor()
a.tell()

> red
> blue

回答3:

Here's the content I did in an ancient answer concerning this feature:

---

The problem you encountered is due to this:

A block is a piece of Python program text that is executed as a unit. The following are blocks: a module, a function body, and a class definition.

(...)

A scope defines the visibility of a name within a block.

(...)

The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods – this includes generator expressions since they are implemented using a function scope. This means that the following will fail:

class A:

   a = 42  

   b = list(a + i for i in range(10))

http://docs.python.org/reference/executionmodel.html#naming-and-binding

The above means: a function body is a code block and a method is a function, then names defined out of the function body present in a class definition do not extend to the function body.

---

It appeared strange to me, when I was reading this, but that's how Python is crafted:

The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods

That's the official documentation that says this.

.

EDIT

heltonbiker wrote an interesting code:

COLOR = 'blue'

class TellColor(object):
    COLOR = 'red'

    def tell(self):
        print self.COLOR   # references class variable
        print COLOR        # references module variable

a = TellColor()
a.tell()

> red
> blue

It made me wonder how the instruction print COLOR written inside the method tell() provokes the printing of the value of the global object COLOR defined outside the class.
I found the answer in this part of the official documentation:

Methods may reference global names in the same way as ordinary functions. The global scope associated with a method is the module containing its definition. (A class is never used as a global scope.) While one rarely encounters a good reason for using global data in a method, there are many legitimate uses of the global scope: for one thing, functions and modules imported into the global scope can be used by methods, as well as functions and classes defined in it. Usually, the class containing the method is itself defined in this global scope (...)

http://docs.python.org/2/tutorial/classes.html#method-objects

When the interpreter has to execute print self.COLOR, as COLOR isn't an instance attribute (that is to say the identifier 'COLOR' doesn't belong to the namespace of the instance), the interpreter goes in the namespace of the class of the instance in search for the identifier 'COLOR' and find it, so it prints the value of TellColor.COLOR

When the interpreter has to execute print COLOR, as there is no attribute access written in this instruction, it will search for the identifier 'COLOR' in the global namespace, which the official documentation says it's the module's namespace.

回答4:

What attribute names are attached to an object (and its class, and the ancestors of that class) is not decidable at compile time. So you either make attribute lookup explicit, or you:

• eradicate local variables (in methods) and always use instance variables. This does no good, as it essentially removes local variables with all their advantages (at least in methods).
• decide whether a base x refers to an attribute or local at runtime (with some extra rules to decide when x = ... adds a new attribute if there's no self.x). This makes code less readable, as you never know which one a name is supposed to be, and essentially turns every local variable in all methods into part of the public interface (as attaching an attribute of that name changes the behavior of a method).

Both have the added disadvantage that they require special casing for methods. Right now, a "method" is just a regular function that happens to be accessible through a class attribute. This is very useful for a wide variety of use good cases.