constructor overwrite

Steve Holden steve at holdenweb.com
Mon Feb 15 15:58:19 CET 2010


Mug wrote:
> hi ,i had a problem on constructor overwrite:
> i have something like:
> 
> class obj:
> def __init__(self, x=100, y=None):
>     if y is None:
>           self.x=x
>     else:
>           self.y=y
> so i can call :
> objet = obj()  # x=100 y=None
> or
> objet = obj(40) # x= 40 y=None
> 
> but if i do :
> objet = obj('not cool') #x='not cool' y=None
> since x is not typed .
> 
> i am waiting for a result:
> objet = obj('not cool') #x=100 y='not cool'
> as they do in C++ or java.
> is there a way to do it?
> thanks

You could check the type(s) of the argument(s) in your code, if you
want, but Python does not support signature analysis, dynamic method
dispatch or static typing. You can do

object = obj("y='not cool')

but I doubt this is what you want. Your post raises a couple of thier
points.

First, __init__ is *not* the constructor. By the time it is called
creation of the new object is already complete, and __init__() (as its
name suggests) merely initializes it. In Python 2's "new-style" classes,
and in Python 3, construction is performed by the class's __new__() method.

Secondly, it seems a little strange that you are happy to create
different instances, in some of which self.y is not initialized and in
others self.x is not initialized. You may have a good reason for doing
this, I merely point it out as a potential cause of AttributeError
exceptions.

regards
 Steve

regards
 Steve

-- 
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