__getattribute__ hook and len() problem

2010-07-15 Thread ernest 
Hi!

I have this class that overrides the __getattribute__ method,
so that it returns the attributes of str(self) instead of the
attributes of self.

class Part(object):
def __init__(self):
self.content = []
def __str__(self):
return str.join('\n', self.content)
def __getattribute__(self, name):
if name in ['content', 'write', '__str__']:
return object.__getattribute__(self, name)
else:
return str(self).__getattribute__(name)
def write(self, data):
self.content.append(data)

Then I do:

In [50]: p = Part()

In [51]: p.write('foo')

In [52]: p.upper()
Out[56]: 'FOO'

This is okay, works as expected.

However, len(p) fails:

TypeError: object of type 'Part' has no len()

And yet, p.__len__() returns 3. I though len(object) simply
called object.__len__.

Can somebody shed some light on this??

Many thanks in advance.

Ernest
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Re: __getattribute__ hook and len() problem

2010-07-15 Thread Chris Rebert 
On Thu, Jul 15, 2010 at 5:42 PM, ernest nfdi...@gmail.com wrote:
 Hi!

 I have this class that overrides the __getattribute__ method,
 so that it returns the attributes of str(self) instead of the
 attributes of self.

 class Part(object):
    def __init__(self):
        self.content = []
    def __str__(self):
        return str.join('\n', self.content)
    def __getattribute__(self, name):
        if name in ['content', 'write', '__str__']:
            return object.__getattribute__(self, name)
        else:
            return str(self).__getattribute__(name)
    def write(self, data):
        self.content.append(data)

 Then I do:

 In [50]: p = Part()

 In [51]: p.write('foo')

snip
 However, len(p) fails:

 TypeError: object of type 'Part' has no len()

 And yet, p.__len__() returns 3. I though len(object) simply
 called object.__len__.

 Can somebody shed some light on this??

Quoth 
http://docs.python.org/reference/datamodel.html#more-attribute-access-for-new-style-classes
:

3.4.2.1. More attribute access for new-style classes

object.__getattribute__(self, name)
snip
***Note: This method may still be bypassed when looking up special
methods as the result of implicit invocation via language syntax or
built-in functions. See Special method lookup for new-style classes
(http://docs.python.org/reference/datamodel.html#new-style-special-lookup
).***
 (emphasis mine)

Cheers,
Chris
--
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Re: __getattribute__ hook and len() problem

2010-07-15 Thread Christian Heimes 
 And yet, p.__len__() returns 3. I though len(object) simply
 called object.__len__.

Not exactly, all __magic__ methods of new style classes are called like
getattr(type(obj), __len__)(obj). As a result magic methods are never
looked up on the object, including hooks like __getattr_() and
__getattribute__().

Christian

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Re: __getattribute__ hook and len() problem

2010-07-15 Thread ernest 
Thanks Chris  Christian.
Mistery solved :)

Ernest
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