classes: need for an explanation

Peter Otten __peter__ at web.de
Thu Jan 18 13:14:07 CET 2007


0k- wrote:

> class Thing(object):
>     props = {}
>     def __init__(self):
>         self.props["text"] = TxtAttr("something important")
> 
> t1 = Thing()
> t2 = Thing()
> 
> t2.props["text"].value = "another string"
> 
> print "t1: %s\nt2: %s" % (t1.props["text"].value,
> t2.props["text"].value)
> 
> the above code outputs:
> 
> t1: another string
> t2: another string
> 
> so the problem i cannot get through is that both t1 and t2 have the
> same attr class instance.
> could somebody please explain me why? :)

Putting an assignment into the class body makes the name a /class/ attribute
which is shared by all instances of the class:

>>> class Thing:
...     props = {} # this is a class attribute
...
>>> t1, t2 = Thing(), Thing()
>>> t1.props is t2.props
True

The solution is to move the attribute creation into the initializer:

>>> class Thing:
...     def __init__(self):
...             self.props = {} # an instance attribute
...
>>> t1, t2 = Thing(), Thing()
>>> t1.props is t2.props
False

Peter




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