Declaring Python class variables

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April 2nd, 2003, 08:14 AM

HarryF

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Declaring Python class variables

Playing around with Python for the first time and have a question about classes - how to you declare class variables? I know you don't have to but I'd like to to make the class easier to read.

In PHP, for example, I might do this;

Code:

class Test {
    var $message; // declare variable
    function Test ($message) {
        $this->message=$message;
    }

    function get() {
        return $this->message;
    }
}

In Python I've got as far as;

Code:

class Test:
    def __init__(self,message):
        self.message=message

    def get(self):
        return self.message

I'd like to be able to declare the message variable to make it readable.

One other question; when declaring class methods, do I always have to make the first parameter "self"? And why is it Python does this in the first place? Seems a little odd compared to methods in Java or PHP for example.

Many thanks.

April 5th, 2003, 10:05 AM

OB_redemption

Contributing User

Join Date: Jan 2002

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>> how to you declare class variables?

You mean instance variables (data members)? Like this:

Code:

class Test:
    my_attr = '';
    def __init__(self,message):
        self.message=message

    def get(self):
        return self.message

Or you can just use self.message like you showed and the instance variable will spring into existence.

To define private variables, add 2 leading underscores (e.g. __myvar) and Python will mangle it into _classname__myvar. Data hiding in this case is not strictly enforced as in Java (since you can access it using '_classname__myvar' - it's more of a 'contractual agreement' basis)

>> when declaring class methods, do I always have to make the first parameter "self"? And why is it Python does this in the first place?

Consider an object x of class Test (which you defined above). Typically, from outside the class, you'd call the get() method of x like so:

Code:

msg = x.get();

What Python does is translate it into

Code:

msg = Test.get(x);

Why the parameter is called 'self' is a matter of convention (meaning you can use something like 'pinkgoblin' instead of 'self', but it's good to follow the convention to prevent confusing others)

April 6th, 2003, 08:53 AM

HarryF

Junior Member

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Many thanks. Clears up alot.

Declaring Python class variables

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