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Python 2.7 is different because all classes are automatically new-style classes.
In Python 2.7 you would have to overwrite __getattribute__() but I am on Python 2.3 here.
And I have to stay on Python 2.3 for now…welcome to hell :-)

Andreas
On 7 Nov 2016, at 13:23, Michael Howitz wrote:

> Am 07.11.2016 um 13:01 schrieb Andreas Jung <lists@zopyx.com>:
>> I have a large Python 2.3 based installation with 200k LOC. As part of a migration project I need to intercept all attribute lookups of all old-style class.
>>
>> Old legacy code:
>>
>> class Foo(Bar):
>> ...
>>
>>
>> My idea is to inject a common mixin class like
>>
>> class Foo(Bar, Mixin):
>> ...
>>
>> class Mixin:
>>
>> def __getattr__(self, k)
>> print repr(self), k
>> return Foo.__getattr__(self, k)
>
> Hi Andreas,
>
> I doubt that your approach works. Attributes defined on `Foo` are _not_ resolved using __getattr__.
> See the following example using Python 2.7:
>
> In [1]: 1 class A:
> 2 x = 'foo'
> 3 def __getattr__(self, key):
> 4 return 'bar'
>
> In [2]: a = A()
>
> In [3]: a.x
> Out[3]: 'foo'
>
> In [4]: a.b = 'b'
>
> In [5]: a.b
> Out[5]: 'b'
>
> Maybe I am missing something here.
>
> --
> Mit freundlichen Grüßen
> Michael Howitz
>

On Mon, Nov 7, 2016, at 14:01, Andreas Jung wrote:
> Python 2.7 is different because all classes are automatically new-style
> classes.
> In Python 2.7 you would have to overwrite __getattribute__() but I am on
> Python 2.3 here.

No :)

In Python 3 all classes are new-style classes. In Python 2 (including
2.7) you have to be explicit and either inherit from object (new-style)
or not (old-style).

In Python 2.7 you get:

>>> class A: pass
>>> class B(object): pass
>>> type(A)
<type 'classobj'>
>>> type(B)
<type 'type'>

In Python 3 both are of type 'type'.

And only new-style classes use '__getattribute__'. '__getattr__' is the
right approach for old-style classes in any Python 2 version, even in
Python 2.7.

Hanno
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Hi Andreas,

As Hanno said, you'll still be in old-style-classes-land even in Python 2.7
unless you inherit from `object` directly.

You said:

My idea is to inject a common mixin class like
> class Foo(Bar, Mixin):
> ...
> class Mixin:
> def __getattr__(self, k)
> print repr(self), k
> return Foo.__getattr__(self, k)


Your recursion error is caused by calling `Foo.__getattr__(self, k)` from
`Mixin`, but `Foo` inherits from `Mixin`. Calling a method of a subclass
from the same-named method in the superclass is a recurssion.

Instead `Mixin` needs to call `Bar.__getattr__` since `Bar` is the
superclass to `Foo`.

Alternatively, if you don't want to name any specific superclass inside the
mixin method, (and since you can't use `super()` since you're using
old-style classes), I suggest a different approach. Instead of a mixin
class, use a method factory, like this:

def my__getattr__factory(SuperClass):
> def __getattr__(self, k):
> print repr(self), k
> return SuperClass.__getattr__(self, k)


Which you can then use as:

class Foo(Bar): # <- No mixin


> __getattr__ = my_getattr__generator(Bar) # <- Bar, not Foo


Regards,

Leo

On 7 November 2016 at 12:29, Hanno Schlichting <hanno@hannosch.eu> wrote:

> On Mon, Nov 7, 2016, at 14:01, Andreas Jung wrote:
> > Python 2.7 is different because all classes are automatically new-style
> > classes.
> > In Python 2.7 you would have to overwrite __getattribute__() but I am on
> > Python 2.3 here.
>
> No :)
>
> In Python 3 all classes are new-style classes. In Python 2 (including
> 2.7) you have to be explicit and either inherit from object (new-style)
> or not (old-style).
>
> In Python 2.7 you get:
>
> >>> class A: pass
> >>> class B(object): pass
> >>> type(A)
> <type 'classobj'>
> >>> type(B)
> <type 'type'>
>
> In Python 3 both are of type 'type'.
>
> And only new-style classes use '__getattribute__'. '__getattr__' is the
> right approach for old-style classes in any Python 2 version, even in
> Python 2.7.
>
> Hanno
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>