Memory error due to big input file

[Steven D'Aprano]

On Mon, 13 Jul 2009 14:20:13 -0700, Aaron Scott wrote:

>> BTW, you should derive all your classes from something.  If nothing
>> else, use object.
>>   class textfile(object):
> 
> Just out of curiousity... why is that? I've been coding in Python for a
> long time, and I never derive my base classes. What's the advantage to
> deriving them?

"Old style" classes (those whose base classes aren't derived from 
anything) have a few disadvantages:


(1) Properties don't work correctly:

>>> class Parrot:  # old-style class
...     def __init__(self):
...             self._x = 3
...     def _setter(self, value):
...             self._x = value
...     def _getter(self):
...             print "Processing ..."
...             return self._x + 1
...     x = property(_getter, _setter)
...
>>> p = Parrot()
>>> p.x
Processing ...
4
>>> p.x = 2
>>> p.x
2

In general, anything that uses the descriptor protocol, not just 
property, will fail to work correctly with old-style classes.


(2) Classes using multiple inheritance with diamond-shaped inheritance 
will be broken.

(3) __slots__ is just an attribute.

(4) super() doesn't work.

And, depending on whether you consider this a disadvantage or an 
advantage:

(5) Special methods like __len__ can be over-ridden on the instance, not 
just the class:

>>> class K:
...     def __len__(self):
...             return 0
...
>>> k = K()
>>> len(k)
0
>>> k.__len__ = lambda : 42
>>> len(k)
42



In their favour:

(1) Less typing.

(2) If you're not using descriptors, including property(), or multiple 
inheritance with diamond diagrams, they work fine.

(3) They're (apparently) a tiny bit faster.



-- 
Steven