I tried to run the following piece of code:
Python 2.3.4 (#53, May 25 2004, 21:17:02) [MSC v.1200 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information.
o = object() o.a = 5
Traceback (most recent call last): File "<stdin>", line 1, in ? AttributeError: 'object' object has no attribute 'a'
But if I do:
class c(object): pass o = c() o.a = 5
...then it, of course, works. So what's wrong with the first example?
It's an instance of class object. ;)
If my memory serves me right, instances of object have no __dict__.
py> o = object() py> o.__dict__ Traceback (most recent call last): File "<interactive input>", line 1, in ? AttributeError: 'object' object has no attribute '__dict__'
Yup, that looks right. As I understand it, the reason for this is so that classes which *really* need to be memory efficient can define __slots__ instead and save some overhead.
If you're interested in having a builtin object that can be used as above, you should help me rally for my Bunch type PEP. They haven't given me a PEP number for it yet, but a patch is available[1] and I've included the current draft of the PEP below.
[1]http://sourceforge.net/tracker/?func=detail&atid=305470&aid=1094542&group_id=5470
Steve
---------------------------------------------------------------------- PEP: XXX Title: Generic Object Data Type Version: $Revision: 1.0 $ Last-Modified: $Date: 2004/11/29 16:00:00 $ Author: Steven Bethard <[EMAIL PROTECTED]> 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 collections module of 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, data-only 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
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.
Examples =========
Converting an XML DOM tree into a tree of nested Bunch objects::
>>> import xml.dom.minidom
>>> def getbunch(element):
... result = Bunch()
... if element.attributes:
... result.update(element.attributes.items())
... children = {}
... for child in element.childNodes:
... if child.nodeType == xml.dom.minidom.Node.TEXT_NODE:
... children.setdefault('text', []).append(
... child.nodeValue)
... else:
... children.setdefault(child.nodeName, []).append(
... getbunch(child))
... result.update(children)
... return result
...
>>> 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 = getbunch(doc.documentElement)
>>> b.a[0].b[1]
Bunch(attr_b=u'3', text=[u' b text '])
Reference Implementation ========================
The code is available as SourceForge patch 1094542 [1]_.
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 ==========
.. [1] http://sourceforge.net/tracker/index.php?func=detail&aid=1094542&group_id=5470&atid=305470
.. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 End: -- http://mail.python.org/mailman/listinfo/python-list
