Newbie question about Class

[JStoneGT at aol.com]

I'm reading the book of "Dive into Python" and got these code pieces:
 
class  UserDict:                                 
def __init__(self,  dict=None):              self.data = {} 
if dict is not None: self.update(dict)
 
My question is,for the statement of:
 
if dict is not None: self.update(dict)
 
Why it's not this one below?
 
if dict is not None: self.data.update(dict)
 
Thanks.
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