__getattribute__ and __slots__

[pascal.parent]

Hi,
I try to define a (new-style) class who:
- have a __slots__ defined to be strict attributes,
- return None if the attribute is 'ok' but not set, or raise a 'normal'
error if the attribute isn't in __slots__.

This code runs, but is it the good way?

Thanks.

class test(object):
__slots__ = ['id']
def __getattr__(self, attr):
if not attr in self.__slots__: raise AttributeError
try:
return self.attr
except:
return None

 

[Peter Otten]

(e-mail address removed) wrote:

Python has both __getattribute__() and __getattr__(). While
__getattribute__() will always be called if you ask for an attribute
__getattr__() serves only as fallback if the attribute if not found by
other means.

I try to define a (new-style) class who:
- have a __slots__ defined to be strict attributes,

__slots__ is a performance/memory optimization. Using __slots__ to introduce
bondage & discipline through the backdoor is against Python's spirit.

- return None if the attribute is 'ok' but not set, or raise a 'normal'
error if the attribute isn't in __slots__.

The approach you have chosen is a very ineffective one. Why don't you just
set the attribute to a default value in the initializer?

This code runs, but is it the good way?

I don't think so...

class test(object):
__slots__ = ['id']
def __getattr__(self, attr):
if not attr in self.__slots__: raise AttributeError
try:
return self.attr

The line above does not do what you think it does. It just calls
test_instance.__getattr__("attr"), and since "attr" is not in __slots__ it
raises an AttributeError.

except:
return None

You should get the same effect with
.... __slots__ = ["id"]
.... def __getattr__(self, name):
.... if name not in self.__slots__:
.... raise AttributeError
.... return 42 # for better visibility
....Traceback (most recent call last):
File "<stdin>", line 1, in ?
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: 'T' object has no attribute 'whatever'

Peter