"""Descriptor utilities.

Utilities to support special Python descriptors [1,2], in particular the use of
a useful pattern for properties we call 'one time properties'.  These are
object attributes which are declared as properties, but become regular
attributes once they've been read the first time.  They can thus be evaluated
later in the object's life cycle, but once evaluated they become normal, static
attributes with no function call overhead on access or any other constraints.

A special ResetMixin class is provided to add a .reset() method to users who
may want to have their objects capable of resetting these computed properties
to their 'untriggered' state.

References
----------
[1] How-To Guide for Descriptors, Raymond
Hettinger. http://users.rcn.com/python/download/Descriptor.htm

[2] Python data model, http://docs.python.org/reference/datamodel.html

Notes
-----
This module is taken from the NiPy project 
(http://neuroimaging.scipy.org/site/index.html), and is BSD licensed.

Authors
-------
- Fernando Perez.
"""

#-----------------------------------------------------------------------------
# Classes and Functions
#-----------------------------------------------------------------------------

class ResetMixin(object):
   """A Mixin class to add a .reset() method to users of OneTimeProperty.

   By default, auto attributes once computed, become static.  If they happen to
   depend on other parts of an object and those parts change, their values may
   now be invalid.

   This class offers a .reset() method that users can call *explicitly* when
   they know the state of their objects may have changed and they want to
   ensure that *all* their special attributes should be invalidated.  Once
   reset() is called, all their auto attributes are reset to their
   OneTimeProperty descriptors, and their accessor functions will be triggered
   again.

   Example
   -------

   >>> class A(ResetMixin):
   ...     def __init__(self,x=1.0):
   ...         self.x = x
   ...
   ...     @auto_attr
   ...     def y(self):
   ...         print '*** y computation executed ***'
   ...         return self.x / 2.0
   ... 

   >>> a = A(10)

   About to access y twice, the second time no computation is done:
   >>> a.y
   *** y computation executed ***
   5.0
   >>> a.y
   5.0

   Changing x
   >>> a.x = 20
   
   a.y doesn't change to 10, since it is a static attribute:
   >>> a.y
   5.0

   We now reset a, and this will then force all auto attributes to recompute
   the next time we access them:
   >>> a.reset()

   About to access y twice again after reset():
   >>> a.y
   *** y computation executed ***
   10.0
   >>> a.y
   10.0
   """

   def reset(self):
      """Reset all OneTimeProperty attributes that may have fired already."""
      instdict = self.__dict__
      classdict = self.__class__.__dict__
      # To reset them, we simply remove them from the instance dict.  At that
      # point, it's as if they had never been computed.  On the next access,
      # the accessor function from the parent class will be called, simply
      # because that's how the python descriptor protocol works.
      for mname, mval in classdict.items():
         if mname in instdict and isinstance(mval, OneTimeProperty):
            delattr(self, mname)


class OneTimeProperty(object):
   """A descriptor to make special properties that become normal attributes.

   This is meant to be used mostly by the auto_attr decorator in this module.
   """
   def __init__(self,func):
       """Create a OneTimeProperty instance.

        Parameters
        ----------
          func : method
          
            The method that will be called the first time to compute a value.
            Afterwards, the method's name will be a standard attribute holding
            the value of this computation.
            """
       self.getter = func
       self.name = func.func_name

   def __get__(self,obj,type=None):
       """This will be called on attribute access on the class or instance. """

       if obj is None:
           # Being called on the class, return the original function. This way,
           # introspection works on the class.
           #return func
           return self.getter

       val = self.getter(obj)
       #print "** auto_attr - loading '%s'" % self.name  # dbg
       setattr(obj, self.name, val)
       return val


def auto_attr(func):
    """Decorator to create OneTimeProperty attributes.

    Parameters
    ----------
      func : method
        The method that will be called the first time to compute a value.
        Afterwards, the method's name will be a standard attribute holding the
        value of this computation.

    Examples
    --------
    >>> class MagicProp(object):
    ...     @auto_attr
    ...     def a(self):
    ...         return 99
    ...     
    >>> x = MagicProp()
    >>> 'a' in x.__dict__
    False
    >>> x.a
    99
    >>> 'a' in x.__dict__
    True
    """
    return OneTimeProperty(func)


