2014-04-23 8:11 GMT+02:00 Cameron Simpson <c...@zip.com.au>:
> On 23Apr2014 09:39, Pavel Volkov <sai...@lists.xtsubasa.org> wrote:
>
>> There are some basics about Python objects I don't understand.
>> Consider this snippet:
>>
>> class X: pass
>>>>>
>>>> ...
>>
>>> x = X()
>>>>> dir(x)
>>>>>
>>>> ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__',
>> '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__',
>> '__init__', '__le__', '__lt__', '__module__', '__ne__', '__new__',
>> '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__',
>> '__str__', '__subclasshook__', '__weakref__']
>>
>>> x.foo = 11
>>>>>
>>>>
>> And now I want to make a simple object in a shorter way, without
>> declaring X class:
>>
>
If you don't want to go through the usual syntax that defines classes you
can use the equivalent code using type to dynamically create a class:
MyClassObject = type('MyClassObject', (object,), dict())
Mind the fact that MyClassObject is a specific kind of object: a class, a
priori, not harmful in anyway
>
>> y = object()
>>>>> dir(y)
>>>>>
>>>> ['__class__', '__delattr__', '__dir__', '__doc__', '__eq__',
>> '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__',
>> '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__',
>> '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
>> '__subclasshook__']
>>
>>> y.foo = 12
>>>>>
>>>> Traceback (most recent call last):
>> File "<stdin>", line 1, in <module>
>> AttributeError: 'object' object has no attribute 'foo'
>>
>> The attribute list is different now and there's no __dict__ and the
>> object does not accept new attributes.
>> Please explain what's going on.
>>
>
> The base "object" class has a fixed set of attributes; you can't add more.
>
Just like any other builtin type: int, float etc... which also means you
can't add method to them dynamically.
Mind the fact that an instance of object is still useful to implement
efficient SENTINEL objects
>
> Almost every other class lets you add attributes, but the price for that
> is that it is slightly in memory footprint and slower to access.
>
class defined in Python except if you define a __slots__
>
> Look up the "__slots__" dunder var in the Python doco index:
>
> https://docs.python.org/3/glossary.html#term-slots
>
> You'll see it as a (rarely used, mostly discouraged) way to force a fixed
> set of attributes onto a class. As with object, this brings a smaller
> memory footprint and faster attribute access, but the price is flexibility.
>
True, still can be the only way to save few MB or... GB without falling
back to C or PyPy.
Have a look at PyPy to how to save memory (and speed things up) without
slots:
http://morepypy.blogspot.fr/2010/11/efficiently-implementing-python-objects.html
In Python 3 there is a class that is equivalent to:
class foo(object):
pass
simple object with a __dict__, I can't find it anymore and also there is
SimpleNamespace<https://docs.python.org/3/library/types.html#types.SimpleNamespace>class.
To sum up:
- If you want to create a sentinel value, use object()
- If you want to create an object to hold values and access them as
attributes, use something like SimpleNamespace
> Cheers,
> Cameron Simpson <c...@zip.com.au>
>
> Try being nothing but bored for 4 hours straight, and then tell me that
> there's no fear involved. - d...@elxr.jpl.nasa.gov (Dave Hayes)
> --
> https://mail.python.org/mailman/listinfo/python-list
>
--
https://mail.python.org/mailman/listinfo/python-list
