QuickNote Descriptor in Python

TL;DR: Descriptor in Python is just getter/setter

Three ways to implement descriptor:

  1. Use a class that implement magic method __get__, __set__, __delete__
  2. Use builtin function property(fget=None, fset=None, fdel=None, doc=None)
  3. Use decorator form of property, like @property def attr(): ..., @attr.setter, @attr.deleter

Make sure all three methods are done at class level rather than at instance level.
(That is, write my_attr = property(...) under class MyClass(object): statement,
but not self.my_attr = property(..) in __init__)

Because Python’s MRO is:

  1. find attr in instance.__dict__ (instance level)
  2. if not found, find attr in instance.__class__.__dict__. If found, try return attr.__get__, otherwise return attr (class level)
  3. if not found, repeat step 2 on instance.__class__.__base__ until attr found or no base class found
  4. if not found, try returning computed attr if __getattr__ is defined

So the descriptor magic is done at class level

Pitfall:

Because descriptor dwells at class level, if one uses descriptor class implementation, all instances may share one common variable.

To fix that:

  1. Hoard one hidden variable in the instance
  2. Use a dictionary in descriptor class to store info of different instances.

The second solution needs the class hashable. (So a meta class is needed to guarantee hashability)

Personal view: To maintain Python’s one simple way to work Pythonists just introduced much more complexity.

Reference:
http://www.ibm.com/developerworks/library/os-pythondescriptors/
http://nbviewer.ipython.org/urls/gist.github.com/ChrisBeaumont/5758381/raw/descriptor_writeup.ipynb
http://docs.python.org/2/howto/descriptor.html

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