posts/lazyproxy-in-python.en.html

   1 <!-- subject: Lazy Proxy in {Python} -->
   2 <!-- date: 2012-07-08 20:56:47 -->
   3 <!-- tags: python, lazy proxy -->
   4 <!-- categories: Articles, Techblog -->
   5
   6 <p>Paths of destiny lead mysterious ways.  Not so long ago, I was
   7   a hard-core C hacker and now, I spend a lot of the time coding in <a
   8   href=/2010/python-wrazenia/>Python</a>.
   9
  10   <p>In somehow related news, I have discovered that my
  11   search-foo is not good enough, when I was unable to find a decent
  12   implementations of several design patterns in Python.
  13
  14   <p>What I needed was a generic proxy that would defer initialisation
  15   of an object to the moment it is first used.  Here is what I came up with:
  16
  17 <pre>
  18 class LazyProxy(object):
  19     __slots__ = '__get'
  20
  21     def __init__(self, cls, *args, **kw):
  22         object.__setattr__(self, '_LazyProxy__get',
  23                            lambda: self.__set(cls(*args, **kw)))
  24
  25     def __set(self, obj):
  26         object.__setattr__(self, '_LazyProxy__get', lambda: obj)
  27         return obj
  28
  29     def __getattr__(self, name):
  30         return getattr(self.__get(), name)
  31
  32     def __setattr__(self, name, value):
  33         return setattr(self.__get(), name, value)
  34
  35     def __delattr__(self, name):
  36         return delattr(self.__get(), name)</pre>
  37
  38 <!-- FULL -->
  39
  40 <p>Here’s how one can use it:
  41
  42   <pre>
  43 class Foo(object):
  44     foo = 'foo'
  45
  46     def __init__(self, bar, baz):
  47         print 'Creating Foo...'
  48         self.foo = 'foobar'
  49         self.bar = bar
  50         self.baz = baz
  51
  52     def a(self):
  53         print 'Foo.a: self=(foo=%s, bar=%s, baz=%s)' % (
  54             self.foo, self.bar, self.baz)
  55
  56     @classmethod
  57     def b(cls):
  58         print 'Foo.b: cls=%s, cls.foo=%s' % (
  59             cls.__name__, cls.foo)
  60
  61     @staticmethod
  62     def c():
  63         print 'Foo.c'
  64
  65 foo = LazyProxy(Foo, 'bar', baz='qux')
  66 print 'LazyProxy created'
  67 foo.a()
  68 foo.b()
  69 foo.c()</pre>
  70
  71   <p><code>foo</code> can be used in (almost) the same
  72   way as an already created <code>Foo</code> object would.  The only
  73   caveat is it will not be an instance of <code>Foo</code> which may or
  74   may not be an issue. Also, it is not thread safe.
  75
  76 <p class=nt>Code © Google Inc.
  77
  78 <!-- COMMENT -->
  79 <!-- date: 2012-07-09 07:07:32 -->
  80 <!-- nick: Radek -->
  81 <!-- nick_url: http://rdominiak.jogger.pl -->
  82
  83 <blockquote>
  84   <p>‘Not so long ago, I was a hard-core C hacker and now, I spend a lot of the time coding in Python.’
  85 </blockquote>
  86
  87 <p>I remember that some time ago you also wrote somethink like ‘why I don’t like Gentoo’, so next post will be ‘Coding Python on Gentoo’? ;)
  88
  89 <!-- COMMENT -->
  90 <!-- date: 2012-07-09 22:09:03 -->
  91 <!-- nick: mina86 -->
  92 <!-- nick_url: http://mina86.com -->
  93
  94 <blockquote>
  95   <p>I remember that some time ago you also wrote somethink like ‘why I don’t like Gentoo’, so next post will be ‘Coding Python on Gentoo’? ;)
  96 </blockquote>
  97
  98 <p>That’s really unlikely. ;) If to anything, I’d probably switch to Debian, but
  99   than again, I’ve never looked at Arch and it was recommended to me a few
 100   times.
 101
 102 <p>True enough though, that no one can tell what future will bring, and there
 103  may be some bizarre event which will make me use Gentoo.
 104
 105 <p>PS. The referenced article: <a href="https://mina86.com/2008/wrabzenia-z-gentoo/">Wrażenia z Gentoo</a> (Polish).