iclassmethod decorator to define both a class and an instance method in one go

You'll find in the logilab.common.decorators module the iclassmethod decorator which may be pretty handy in some cases as it allows methods to be both called as class methods or as instance methods. In the first case the first argument will be the class and the second case it will be the instance.

Example extracted (and adapted for simplicity) from CubicWeb:

from logilab.common.decorators import iclassmethod

class Form(object):
  _fields_ = []

  def __init__(self):
      self.fields = list(self._fields_)

  @iclassmethod
  def field_by_name(cls_or_self, name):
      """return field with the given name and role"""
      if isinstance(cls_or_self, type):
          fields = cls_or_self._fields_
      else:
          fields = cls_or_self.fields
      for field in fields:
          if field.name == name:
              return field
      raise Exception('FieldNotFound: %s' % name)

Example session:

>>> from logilab.common import attrdict
>>> f = Form()
>>> f.fields.append(attrdict({'name': 'something', 'value': 1})
>>> f.field_by_name('something')
{'name': 'something', 'value': 1}
>>> Form.field_by_name('something')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 15, in field_by_name
Exception: FieldNotFound: something

So we get a field_by_name method which will act differently (actually use different input data) when called as instance method or as class method.

Also notice the attrdict trick that can also be achieved with the Python 2.6 named tuple.