Magic?

mk mrkafk at gmail.com
Sun Jul 13 18:40:18 CEST 2008


So I was playing around with properties and wrote this:

class lstr(str):
      def __init__(self, initval):
           self._s = initval
           self._len = len(self._s)

      def fget_s(self):
           return str(self._s)

      def fset_s(self, val):
           self._s = val
           self._len = len(self._s)

      s = property(fget_s, fset_s)

      def fget_len(self):
           return self._len

      def fset_len(self, val):
           raise AttributeError, "Attribute is read-only."

      len = property(fget_len, fset_len)


I obviously aimed at defining setters and getters for 's' and 'len' 
attributes via using properties to that.

However, it appears that somehow this object prints the value of 's' 
attribute without me setting any specific methods to do that:

 >>> astr = lstr('abcdef')
 >>> astr
'abcdef'
 >>> astr.swapcase()
'ABCDEF'

How does it know to do that? I mean, I can understand how it knows to do 
that since I used property:

 >>> astr.s
'abcdef'

 >>> vars(astr)
{'_len': 6, '_s': 'abcdef'}

How does the instance know to use _s value to return when the instance 
is called?

Is this due to some trick handling of overriden __init__ method (i.e. it 
knows to treat initval argument somehow specially)? Some other way? If 
so, how?




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