[Tutor] Class definition confusion

[Mark Lawrence]

On 15/02/2012 18:14, Sivaram Neelakantan wrote:
>
> I was under the impression that you have to define the attributes of
> the class before using it in an instance.  Following the book
> 'thinking in Python',
>
>>>> class Point:
> ...     """pts in 2d space"""
> ...
>>>> print Point
> __main__.Point
>>>> b = Point()
>>>> b.x =3
>>>> b.y =4
>>>> print b.y
> 4
>>>>
>
> Why is it not throwing an error?  This is confusing me a bit.
>
>   sivaram
>   --
>
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Your impression is incorrect.  This type of behaviour is allowed because 
of Python's dynamic nature, so the following is fine.

 >>> class Point:
... 	"""pts in 2d space"""
... 	
 >>> b = Point()
 >>> b.x = 3
 >>> b.y = 4
 >>> del b.x
 >>> del b.y
 >>> b.l = 5
 >>> b.m = 6
 >>> print b, b.l, b.m
<__main__.Point instance at 0x02FB89B8> 5 6

Also be careful of your terminology.  Here we are discussing instance 
attributes.  Class attributes are different in that they are are shared 
at the class level so.

 >>> class Point:
... 	"""pts in 2d space"""
... 	x = 3
... 	y = 4
... 	
 >>> a = Point()
 >>> b = Point()
 >>> a.x
3
 >>> b.y
4

HTH.

-- 
Cheers.

Mark Lawrence.