python object typecasting

Question

Ive been trying search how to pass object reference in python and type cast it similar to Java but no to avail. I duno if this topic exists somewhere here.

My trouble is i have to pass the object reference to a class constructor. But i duno how to typecast the reference to an object. In java though I have accomplish this but i have to transfer the code to the server side.

many thanks, Jack

class SearchRectangle:
    def __init__(self, lower_left_subgrid_x, lower_left_subgrid_y, rectangle_width, rectangle_height):

        self.min_subgrid_x = int(lower_left_subgrid_x)
        self.max_subgrid_x = int(self.min_subgrid_x + rectangle_width -1)
        self.min_subgrid_y = int(lower_left_subgrid_y)
        self.max_subgrid_y = int(self.min_subgrid_y + rectangle_height -1)

     ...blah



class SearchRectangleMultiGrid:
 # parent rectangle should be a SearchRectangle instance 
    def __init__(self, parent_rectangle):
        self.parent_rectangle = SearchRectangle()parent_rectangle



    # test codes
    test_rect = SearchRectangle(test_subgrid.subgrid_x, test_subgrid.subgrid_y, 18, 18)
    print "\n\nTest SearchRectangle";
    print test_rect.to_string()
    print test_rect.sql_clause

    test_rec_multi = SearchRectangleMultiGrid(test_rect)
    print "\n\nTest SearchRectangleMulti"
    test_rec_multi.parent_rectangle.to_string()

Wolph · Accepted Answer · 2010-12-25 03:33:58Z

7

Python is a dynamically typed language and as such, it doesn't make much sense to cast something unless you specifically need it in that type.

In Python you should use Duck Typing instead: http://en.wikipedia.org/wiki/Duck_typing

So instead of trying to convert parent_rectangle to a SearchRectangle() you should simply test if SearchRectangle() has the properties you need.

Or if you really want to be sure that you'll always get a SearchRectangle(), use isinstance like this:

if isinstance(parent_rectangle, SearchRectangle):

This might be a good read for you: http://dirtsimple.org/2004/12/python-is-not-java.html

edited Dec 25, 2010 at 3:33

answered Dec 25, 2010 at 2:19

Wolph

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2 Comments

Jack Shade Over a year ago

Yes, but how can I tell the IDE at design time that an object has a specific type (autocomplete etc ...?

Wolph Over a year ago

It depends on the IDE, but many support the type hints given through the Sphinx rtype parameter: sphinx-doc.org/domains.html?highlight=rtype#info-field-lists And some might even support the Google style Returns option: sphinx-doc.org/ext/…

Falmarri · Accepted Answer · 2010-12-25 02:18:41Z

2

There's no reason to cast anything in python. What are you trying to do? Just use the object like you would, and if it's not of the correct type it will fail. There's no such thing as casting since variable names don't have a type associated with them.

answered Dec 25, 2010 at 2:18

Falmarri

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2 Comments

anaval Over a year ago

super useful for autocomplete

Nilesh Kesar Over a year ago

They actually can have types in python 3 you can specify excatly what it is

Lennart Regebro · Accepted Answer · 2010-12-25 08:54:10Z

0

Further explanation:

Casting is the act of taking a pointer/reference to one type of object, and say to the compiler "Yeah, I know this is a foo reference but please pretend it is a bar reference".

Python do not have pointers/references in that sense (although in another sense, everything is references). Also, the compiler/interpreter doesn't care what the type is in the first place. Hence, casting is both impossible and pointless.

So in your example: Just skip the type casting. It'll work anyway. And if it doesn't. then make a question on that problem.

answered Dec 25, 2010 at 8:54

Lennart Regebro

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