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To: python-list@python.org
From: Robert Kern <robert.kern@gmail.com>
Subject: Re: skipping __init__ and using exploiting a class member instead
Date: Sat, 19 Oct 2013 23:18:10 +0100
References: <fb1b5b6f-619c-4acd-b1bf-a375ccc6f1a6@googlegroups.com> <5262FFED.9010309@nedbatchelder.com>
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On 2013-10-19 22:55, Ned Batchelder wrote:
> On 10/19/13 5:44 PM, Peter Cacioppi wrote:
>> Is the following considered poor Python form?
>>
>> class Foo (object) :
>>      _lazy = None
>>      def foo(self, x) :
>>          _lazy = _lazy or self.get_something(x)
>>      def get_something(self, x) :
>>          # doesn't really matter
>>
>> I like this idiom for certain situations, just wondering if it will raise the
>> hackles of other Pythonistas.
>>
>> I use this idiom sparingly, but sometimes it just fits the task at hand, I
>> hear Guidos voice saying "use the Force" in my ear, etc.
>
> You present this as a choice between __init__ or a class attribute, but those
> two things are very different.  Is your intent to have an instance attribute, or
> a class attribute?  Lazily populated instance attributes are fine, I would do it
> like this:
>
> class Foo(object):
>      def __init__(self):
>          self._lazy = None
>
>      def foo(self, x):
>          if self._lazy is None:
>              self._lazy = self.get_something(x)
>          ...

I think he left some important characters out.

class Foo (object) :
     _lazy = None
     def foo(self, x) :
         self._lazy = self._lazy or self.get_something(x)
         # Use self._lazy for something

     def get_something(self, x) :
         # doesn't really matter

The main difference being that he doesn't initialize the instance attribute and 
just relies on the fallback to class attribute lookup. In my experience, using a 
static[1] class attribute as a default for an instance attribute is accepted 
practice, and one that gets touted as a positive feature of Python's namespace 
model when compared against other languages. That said, I have seen it more 
often in the past.

[1] In the general "unchanging" sense rather than the C++ "static" keyword sense.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco