How-To Guide for Descriptors - The Diigo Meta page

• 12 Nov 09
  

  david miller

  python descriptors

• 06 Nov 09
  

  versionthirteen

  django python tutorial descriptors programming

  • Since staticmethods return the underlying function with no changes, the example
    calls are unexciting:

    
    

    >>> class E(object):
         def f(x):
              print x
         f = staticmethod(f)
    
    >>> print E.f(3)
    3
    >>> print E().f(3)
    3

  • Unlike static methods, class methods prepend the class reference to the
    argument list before calling the function. This format is the same
    for whether the caller is an object or a class:

    
    

    >>> class E(object):
         def f(klass, x):
              return klass.__name__, x
         f = classmethod(f)
    
    >>> print E.f(3)
    ('E', 3)
    >>> print E().f(3)
    ('E', 3)

• 08 Aug 09
  

  Mert Nuhoglu

  çok kaliteli bir makale.

  python metaprogramming

  • RevealAccess
  • If any of those methods are
    defined for an object, it is said to be a descriptor.
  • 26 more annotations...
  • • __get__, __set__, and __delete__
    • efault behavior for attribute access is to get, set, or delete the
      attribute
    • a.x has a lookup chain
      starting with a.__dict__['x'], then type(a).__dict__['x'], and continuing
      through the base classes of type(a) excluding metaclasses
    • If the looked-up value is an object defining one of the
      descriptor methods, then Python may override the default behavior and invoke the
      descriptor method instead.
    • They are the mechanism
      behind properties, methods, static methods, class methods, and super()

    • descr.__get__(self, obj, type=None) --> value

      
      

      descr.__set__(self, obj, value) --> None

      
      

      descr.__delete__(self, obj) --> None

    • Define any of these methods and an object is
      considered a descriptor
    • both __get__ and __set__, it is considered a data
      descriptor.
    • only define __get__ are called non-data
      descriptors
    • read-only data descriptor, define both __get__ and __set__
      with the __set__ raising an AttributeError when called
    • the machinery is in object.__getattribute__ which transforms
      b.x into type(b).__dict__['x'].__get__(b, type(b))
    • a precedence chain that gives data descriptors priority over instance
      variables, instance variables priority over non-data descriptors, and assigns
      lowest priority to __getattr__ if provided
    • • descriptors are invoked by the __getattribute__ method
      • overriding __getattribute__ prevents automatic descriptor calls
    • RevealAccess
    • Several use cases are
      so common that they have been packaged into individual function calls.
    • a data descriptor that triggers
      function calls upon access to an attribute
    • property(fget=None, fset=None, fdel=None, doc=None) -> property attribute
    • class C(object):
      def getx(self): return self.__x
      def setx(self, value): self.__x = value
      def delx(self): del self.__x
      x = property(getx, setx, delx, "I'm the 'x' property.")
    • recalculated on every access
    • class Cell(object):
      . . .
      def getvalue(self, obj):
      "Recalculate cell before returning value"
      self.recalc()
      return obj._value
      value = property(getvalue)
    • Class dictionaries store methods as functions
    • __get__ method for binding
      methods during attribute access.

      • 

        Mert Nuhoglu on 2009-08-08

        fonksiyonlar birer obje olduğundan buna ihtiyaç var. javada buna gerek olmuyor, çünkü fonksiyonlar obje değil.
        fonksiyon bağımsız bir instance olduğundan, hangi classa bağlı olduğunu bilmesi gerekir.
    • bound and unbound methods are two different types.
    • f(klass, *args)
    • C.f is the equivalent of a direct lookup into
      object.__getattribute__(c, "f") or object.__getattribute__(C, "f")
    • def __get__(self, obj, objtype=None):
      return self.f
• 20 May 09
  

  Gavin Freeman

  python advance

• 27 Feb 09
  

  Doug Y'barbo

  Python CS CoreDistribution OOP

  • __get__, __set__, and __delete__. If any of those methods are
    defined for an object, it is said to be a descriptor.
  • The default behavior for attribute access is to get, set, or delete the
    attribute from an object's dictionary
  • 7 more annotations...
  • • descr.__get__(self, obj, type=None) --> value
    • descr.__set__(self, obj, value) --> None
    • descr.__delete__(self, obj) --> Non
    • If an object defines both __get__ and __set__, it is considered a data
      descriptor
    • escriptors that only define __get__ are called non-data
      descriptor
    • A descriptor can be called directly by its method name
    • descriptors are invoked by the __getattribute__ method
• 15 Oct 08
  

  reckoner reckoner

  descriptors

  python descriptors descriptor programming tutorial tutorials howto

• 13 Sep 08
  

  Terry Jones

  python descriptors

• 11 Sep 08
  

  Stoyan Shopov

  python

• 12 Aug 08
  

  Jean-Lou Dupont
  • a descriptor is an object attribute with "binding behavior", one whose
    attribute access has been overridden by methods in the descriptor protocol.
  • descriptors are invoked by the __getattribute__ method
  • 4 more annotations...
  • • object.__getattribute__ and type.__getattribute__ make different
      calls to __get__.
    • The property() builtin helps whenever a user interface has granted attribute
      access and then subsequent changes require the intervention of a method.
    • Class dictionaries store methods as functions. In a class definition, methods
      are written using def and lambda, the usual tools for creating
      functions. The only difference from regular functions is that the first
      argument is reserved for the object instance. By Python convention, the
      instance reference is called self but may be called this or any
      other variable name.
    • Good candidates for static methods are methods that do not reference the
      self variable.
• 01 Apr 08
  

  j p
  • __get__, __set__, and __delete__. If any of those methods are
    defined for an object, it is said to be a descriptor.
  • descriptors are only invoked
    for new style objects or classes
  • 7 more annotations...
  • • They are the mechanism
      behind properties, methods, static methods, class methods, and super().

    • Calling property() is a succinct way of building a data descriptor that triggers
      function calls upon access to an attribute. Its signature is:

      
      

      property(fget=None, fset=None, fdel=None, doc=None) -> property attribute

    • all functions are non-data
      descriptors which return bound or unbound methods depending whether they are
      invoked from an object or a class
    • the effects of calling a method object depend on the im_self field.
      If set (meaning bound), the original function (stored in the im_func field) is
      called as expected with the first argument set to the instance. If unbound, all
      of the arguments are passed unchanged to the original function.
    • functions have a __get__ method so that they can be converted to a
      method when accessed as attributes.
    • Calling klass.f(*args) becomes
      f(*args)
    • class methods prepend the class reference to the
      argument list before calling the function
• 02 Jan 08
  

  apcitdx

  python

• 01 Jun 07
  

  Carlos Santos

  code article design development oop python reference software guide howto theory Languages descriptors metadata metaprogramming property decorators

• 24 Sep 06
  

  Greg

  descriptors new-style python

• 04 Aug 06
  

  Kazuo Moriwaka

  descriptor doc howto programming python

• 26 Jan 05
  

  Luc Stepniewski

  python descriptor internal usage language

Public Stiky Notes

• 
  Mert Nuhoglu on 2009-08-08
  fonksiyonlar birer obje olduğundan buna ihtiyaç var. javada buna gerek olmuyor, çünkü fonksiyonlar obje değil.
  fonksiyon bağımsız bir instance olduğundan, hangi classa bağlı olduğunu bilmesi gerekir.

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