[Tutor] instance variables and the instance dictionary

[Magnus Lycka]

At 09:54 2003-01-09 -0700, Poor Yorick wrote:
>In an instance, are the following statements equivalent?  Are there any 
>caveats to setting an instance variable the second way?
>
>self.var1 = 5
>
>self.__dict__.__setitem__('var1', 5)

As others have mentioned, manipulating the __dict__ bypasses
__setattr__. A consequence of this is that the code in an
implementation of __setattr__ often uses __dict__ manipulation
to avoid infinite recursion.

A method like below would call itself until the stack breaks.

    def __setattr__(self, attr, val):
       # do some error checks
       ...
       setattr(self, attr, val)

You need to do:

    def __setattr__(self, attr, val):
       # do some error checks
       ...
       self.__dict__[attr] = val

instead.

But note!

DANGER!!!

What will happen if attr is '__private' ???



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Magnus Lycka, Thinkware AB
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