inherit without calling parent class constructor?

Steven Bethard steven.bethard at gmail.com
Wed Jan 26 20:45:42 CET 2005


Christian Dieterich wrote:
> The size attribute only needs to be computed once and stays constant 
> after that. The lazy property recipe of Scott David Daniels looks 
> promising. I'll try that, when I've installed Python 2.4. However, I 
> need my package to work on machines where there is Python 2.2 and 2.3 only.

Note that:
     @deco
     def func(...):
         ...
is basically just syntactic sugar for:
     def func(...):
         ...
     func = deco(func)
So you don't need 2.4 to use the recipe; you just need a version of 
Python that supports descriptors (2.2 or later) since the recipe uses a 
class with __get__.  I believe the following code should work for Python 
2.2 and later, though I only have 2.4 to test it on at home.

py> class LazyAttribute(object):
...     def __init__(self, calculate_function):
...         self._calculate = calculate_function
...     def __get__(self, obj, _=None):
...         if obj is None:
...             return self
...         value = self._calculate(obj)
...         setattr(obj, self._calculate.func_name, value)
...         return value
...
py> class B(object):
...     def __init__(self, length):
...         self._length = length
...     def size(self):
...         print 'some expensive calculation'
...         return self._length
...     size = LazyAttribute(size)
...
py> class D(B):
...     def __init__(self, length):
...         super(D, self).__init__(length)
...         self.value = 1
...
py> b = B(5)
py> b.size
some expensive calculation
5
py> b.size
5
py> d = D(6)
py> d.size
some expensive calculation
6
py> d.size
6

Note that b.size and d.size are only calculated once each, and if d.size 
is never accessed, you'll never incur the costs of calculating it.

Steve



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