pre-PEP generic objects

Steven Bethard steven.bethard at gmail.com
Tue Nov 30 00:19:39 CET 2004


I promised I'd put together a PEP for a 'generic object' data type for 
Python 2.5 that allows one to replace __getitem__ style access with 
dotted-attribute style access (without declaring another class).  Any 
comments would be appreciated!

Thanks!

Steve

----------------------------------------------------------------------
Title: Generic Object Data Type
Version: $Revision: 1.0 $
Last-Modified: $Date: 2004/11/29 16:00:00 $
Author: Steven Bethard <steven.bethard at gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 29-Nov-2004
Python-Version: 2.5
Post-History: 29-Nov-2004


Abstract
========

This PEP proposes a standard library addition to support the simple
creation of 'generic' objects which can be given named attributes
without the need to declare a class. Such attribute-value mappings are
intended to complement the name-value mappings provided by Python's
builtin dict objects.


Motivation
==========

Python's dict objects provide a simple way of creating anonymous
name-value mappings. These mappings use the __getitem__ protocol to
access the value associated with a name, so that code generally appears
like::

     mapping['name']

Occasionally, a programmer may decide that dotted-attribute style access
is more appropriate to the domain than __getitem__ style access, and
that their mapping should be accessed like::

     mapping.name

Currently, if a Python programmer makes this design decision, they are
forced to declare a new class, and then build instances of this class.
When no methods are to be associated with the attribute-value mappings,
declaring a new class can be overkill.  This PEP proposes adding a
simple type to the standard library that can be used to build such
attribute-value mappings.

Providing such a type allows the Python programmer to determine which
type of mapping is most appropriate to their domain and apply this
choice with minimal effort.  Some of the suggested uses include:


Returning Named Results
-----------------------

It is often appropriate for a function that returns multiple items to
give names to the different items returned.  The type suggested in this
PEP provides a simple means of doing this that allows the returned
values to be accessed in the usual attribute-style access::

     >>> def f(x):
     ...     return Bunch(double=2*x, squared=x**2)
     ...
     >>> y = f(10)
     >>> y.double
     20
     >>> y.squared
     100


Representing Hierarchical Data
------------------------------

The type suggested in this PEP also allows a simple means of
representing hierarchical data that allows attribute-style access::

     >>> x = Bunch(spam=Bunch(rabbit=1, badger=[2, 3, 4]), ham='neewom')
     >>> x.spam.badger
     [2, 3, 4]
     >>> x.ham
     'neewom'


Rationale
=========

As Bunch objects are intended primarily to replace simple classes,
simple Bunch construction was a primary concern.  As such, the Bunch
constructor supports creation from keyword arguments, dicts, and
sequences of (attribute, value) pairs::

     >>> Bunch(eggs=1, spam=2, ham=3)
     Bunch(eggs=1, ham=3, spam=2)
     >>> Bunch({'eggs':1, 'spam':2, 'ham':3})
     Bunch(eggs=1, ham=3, spam=2)
     >>> Bunch([('eggs',1), ('spam',2), ('ham',3)])
     Bunch(eggs=1, ham=3, spam=2)

To allow attribute-value mappings to be easily combined, the update
method of Bunch objects supports similar arguments.

If Bunch objects are used to represent hierarchical data, comparison of
such objects becomes a concern.  For this reason, Bunch objects support
object equality::

     >>> x = Bunch(parrot=Bunch(lumberjack=True, spam=42), peng='shrub')
     >>> y = Bunch(peng='shrub', parrot=Bunch(spam=42, lumberjack=True))
     >>> z = Bunch(parrot=Bunch(lumberjack=True), peng='shrub')
     >>> x == y
     True
     >>> x == z
     False

Additionally, to allow users of the Bunch type to convert other
hierarchical data into Bunch objects, a frommapping classmethod is
supported.  This can be used, for example, to convert an XML DOM tree
into a tree of nested Bunch objects::

     >>> import xml.dom.minidom
     >>> def getitems(element):
     ...     if not isinstance(element, xml.dom.minidom.Element):
     ...         raise TypeError('items only retrievable from Elements')
     ...     if element.attributes:
     ...         for key, value in element.attributes.items():
     ...             yield key, value
     ...     children = {}
     ...     for child in element.childNodes:
     ...         if child.nodeType == xml.dom.minidom.Node.TEXT_NODE:
     ...             text_list = children.setdefault('text', [])
     ...             text_list.append(child.nodeValue)
     ...         else:
     ...             children.setdefault(child.nodeName, []).append(
     ...                 Bunch.frommapping(child, getitems=getitems))
     ...     for name, child_list in children.items():
     ...         yield name, child_list
     ...
     >>> doc = xml.dom.minidom.parseString("""\
     ... <xml>
     ...   <a attr_a="1">
     ...     a text 1
     ...     <b attr_b="2" />
     ...     <b attr_b="3"> b text </b>
     ...     a text 2
     ...   </a>
     ...   <c attr_c="4"> c text </c>
     ... </xml>""")
     >>> b = Bunch.frommapping(doc.documentElement, getitems=getitems)
     >>> b.a[0].b[1]
     Bunch(attr_b=u'3', text=[u' b text '])

Note that support for the various mapping methods, e.g.
__(get|set|del)item__, __len__, __iter__, __contains__, items, keys,
values, etc. was intentionally omitted as these methods did not seem to
be necessary for the core uses of an attribute-value mapping.  If such
methods are truly necessary for a given use case, this may suggest that
a dict object is a more appropriate type for that use.


Reference Implementation
========================

(This will be replaced with a link to a SF patch when I think I've
made all the necessary corrections)::

     import operator as _operator

     class Bunch(object):
         """Bunch([bunch|dict|seq], **kwds) -> new bunch with specified
         attributes

         The new Bunch object's attributes are initialized from (if
         provided) either another Bunch object's attributes, a
         dictionary, or a sequence of (name, value) pairs, then from the
         name=value pairs in the keyword argument list.

         Example Usage:
         >>> Bunch(eggs=1, spam=2, ham=3)
         Bunch(eggs=1, ham=3, spam=2)
         >>> Bunch({'eggs':1, 'spam':2, 'ham':3})
         Bunch(eggs=1, ham=3, spam=2)
         >>> Bunch([('eggs',1), ('spam',2), ('ham',3)])
         Bunch(eggs=1, ham=3, spam=2)
         >>> Bunch(Bunch(eggs=1, spam=2), ham=3)
         Bunch(eggs=1, ham=3, spam=2)
         """

         def __init__(self, *args, **kwds):
             """Initializes a Bunch instance."""
             self.update(*args, **kwds)

         def __eq__(self, other):
             """x.__eq__(y) <==> x == y"""
             return (isinstance(other, self.__class__)
                     and self.__dict__ == other.__dict__)

         def __repr__(self):
             """x.__repr__() <==> repr(x)

             If all attribute values in this bunch (and any nested
             bunches) are reproducable with eval(repr(x)), then the Bunch
             object is also reproducable for eval(repr(x)).
             """
             return '%s(%s)' % (self.__class__.__name__,
                                ', '.join('%s=%r' % (k, v)
                                          for k, v
                                          in self.__dict__.items()))

         def update(self, *args, **kwds):
             """update([bunch|dict|seq], **kwds) -> None

             Updates a Bunch object's attributes from (if provided)
             either another Bunch object's attributes, a dictionary, or a
             sequence of (name, value) pairs, then from the name=value
             pairs in the keyword argument list.
             """
             if len(args) == 1:
                 other, = args
                 if isinstance(other, self.__class__):
                     other = other.__dict__
                 try:
                     self.__dict__.update(other)
                 except TypeError:
                     raise TypeError('cannot update Bunch with %s' %
                                     type(other).__name__)
             elif len(args) != 0:
                 raise TypeError('expected 1 argument, got %i' %
                                 len(args))
             self.__dict__.update(kwds)

         @classmethod
         def frommapping(cls, mapping, getitems=None):
             """Create a Bunch object from a (possibly nested) mapping.

             Note that, unlike the Bunch constructor, frommapping
             recursively converts all mappings to bunches.

             Example Usage:
             >>> Bunch.frommapping({'eggs':1,
             ...                    'spam':{'ham':2, 'badger':3}})
             Bunch(eggs=1, spam=Bunch(ham=2, badger=3))

             Keyword Arguments:
             mapping -- a mapping object
             getitems -- a function that takes the mapping as a parameter
                 and returns an iterable of (key, value) pairs.  If not
                 provided, the items method on the mapping object will be
                 used, or (key, mapping[key]) values will be generated if
                 the mapping object does not provide an items method.

             Note that getitems will be applied recursively to each value
             in the mapping.  It should raise a TypeError if it is
             applied to an object for which it cannot produce
             (key, value) pairs.
             """
             # determine which items() method to use
             if getitems is None:
                 try:
                     getitems = type(mapping).items
                 except AttributeError:
                     getitems = _items
             # build the Bunch from the mapping, recursively
             result = cls()
             for key, value in getitems(mapping):
                 try:
                     value = cls.frommapping(value, getitems=getitems)
                 except TypeError:
                     pass
                 setattr(result, key, value)
             return result


     def _items(mapping):
         """Produces (key, value) pairs from a mapping object.

         Intended for use with mapping objects that do not supply an
         items method.
         """
         for key in mapping:
             yield key, mapping[key]


Open Issues
===========
What should the type be named?  Some suggestions include 'Bunch',
'Record' and 'Struct'.

Where should the type be placed?  The current suggestion is the
collections module.


References
==========



..
    Local Variables:
    mode: indented-text
    indent-tabs-mode: nil
    sentence-end-double-space: t
    fill-column: 70
    End:



More information about the Python-list mailing list