Re: [Tutor] Contructor Overloading and Function Tooktips

Brian van den Broek Tue, 19 Apr 2005 11:33:16 -0700

Gooch, John said unto the world upon 2005-04-19 10:20:

Brian,

I think in the OO world it is called Polymorphism, where you have a single
function name, but multiple definitions that are distinguished from one
another by the number of arguments, type of arguments, and sometimes (
Smalltalk ) the return type of the function.

Here are some example function declarations ( C++ style )
boolean greaterThan( int A, int B );
boolean greaterThan( char A, char B );
boolean greaterThan( double A, double B );

All of these functions are called as "greaterThan( A,B )" and all of them
return a boolean "true" or "false" value, but the compiler decides which one
of the three functions above gets called depending on the data type of A and
B. My first question was whether or not you can do this in Python with the
__init__ function. In C++ you can have multiple contructors for a class,
with the arguments deciding which contructor is called. Here is an example:

class Circle : Shape {
public:
        Circle();//creates default circle object

        Circle( int x, int y, float radius ); //creates circle object with
specified x,y coordinates and radius
};

now, replace 'Circle' in the 'public:' area with '__init__' and you have a picture of what I would like to do in Python, but I don't know if the language support it or not.

Thank you for answering the second part of the question, I will try that
technique out.


Hi John,

OK, I think I see what you meant.

First thing is, while I understand it was just an example, your greaterThan examples are handled differently in Python, I think. There are "special methods" for stipulating how the objects of a class behave when passed to the standard comparisons (like '>'). They are all methods with double leading and trailing underscores such as __gt__ I'm typing off line, but searching the docs for "special method" should get you there. (Also, if you have it, p.92 of Python in a Nutshell covers them.)

OK, but that was just your example :-) Here is some ugly quick code which might show you how to meet you needs:

class JohnsMultiInitClass:
    def __init__(self, first, second):
        if type(first) == type(second) == str:
            self.string_init(first, second)
        if type(first) == type(second) == int:
            self.int_init(first, second)
    def string_init(self, first, second):
        # string init code here
    def int_init(self, first, second):
        # int init code here

That said, I'm not so sure this is a good thing. I'm no pro, still getting an handle on OOP, and dangerous only in Python. But, I suspect that this is not the Python way to do this sort of thing. (Perhaps people with C++ experience can weigh in to confirm or deny.)

Best,

Brian vdB

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Re: [Tutor] Contructor Overloading and Function Tooktips

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