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Python class inherits object

tjvr
1#
tjvr Published in 2010-10-25 14:15:26Z

Is there any reason for a class declaration to inherit from object?

I just found some code that does this and I can't find a good reason why.

class MyClass(object):
    # class code follows...
Jim Fasarakis Hilliard
2#
Jim Fasarakis Hilliard Reply to 2017-12-16 18:26:59Z

Is there any reason for a class declaration to inherit from object?

In Python 3, apart from compatibility between Python 2 and 3, no. In Python 2, many reasons.


Python 2.x story:

In Python 2.x (from 2.2 onwards) there's two styles of classes depending on the presence or absence of a built-in type as a base-class:

  1. "classic" style classes: they have no built-in type as a base class:

    >>> class ClassicSpam:      # no base class
    ...     pass
    >>> ClassicSpam.__bases__
    ()
    
  2. "new" style classes: they have a built-in type as a base class meaning that, directly or indirectly, they have object as a base class:

    >>> class NewSpam(object):           # directly inherit from object
    ...    pass
    >>> NewSpam.__bases__
    (<type 'object'>,)
    >>> class IntSpam(int):              # indirectly inherit from object...
    ...    pass
    >>> IntSpam.__bases__
    (<type 'int'>,) 
    >>> IntSpam.__bases__[0].__bases__   # ... because int inherits from object  
    (<type 'object'>,)
    

Without a doubt, when writing a class you'll always want to go for new-style classes. The perks of doing so are numerous, to list some of them:

  • Support for descriptors. Specifically, the following constructs are made possible with descriptors:

    1. classmethod: A method that receives the class as an implicit argument instead of the instance.
    2. staticmethod: A method that does not receive the implicit argument self as a first argument.
    3. properties with property: Create functions for managing the getting, setting and deleting of an attribute.
    4. __slots__: Saves memory consumptions of a class and also results in faster attribute access. Of course, it does impose limitations.
  • The __new__ static method: lets you customize how new class instances are created.

  • Method resolution order (MRO): in what order the base classes of a class will be searched when trying to resolve which method to call.

  • Related to MRO, super calls. Also see, super() considered super.

If you don't inherit from object, forget these. A more exhaustive description of the previous bullet points along with other perks of "new" style classes can be found here.

One of the downsides of new-style classes is that the class itself is more memory demanding. Unless you're creating many class objects, though, I doubt this would be an issue and it's a negative sinking in a sea of positives.


Python 3.x story:

In Python 3, things are simplified. Only new-style classes exist (referred to plainly as classes) so, the only difference in adding object is requiring you to type in 8 more characters. This:

class ClassicSpam:
    pass

is completely equivalent (apart from their name :-) to this:

class NewSpam(object):
     pass

and to this:

class Spam():
    pass

all have object in their __bases__.

>>> [object in cls.__bases__ for cls in {Spam, NewSpam, ClassicSpam}]
[True, True, True]

So, what should you I do?

In Python 2: always inherit from object explicitly. Get the perks.

In Python 3: inherit from object if you are writing code that tries to be Python agnostic, that is, it needs to work both in Python 2 and in Python 3. Otherwise don't, it really makes no difference.

SLaks
3#
SLaks Reply to 2010-10-25 14:20:09Z

This creates a new-style class.

knitti
4#
knitti Reply to 2010-10-25 14:20:34Z

Yes, it's historical. Without its old-style classes.

If you use type() on an old-style object, you just get "instance". On a new-style object you get its class

Jim Fasarakis Hilliard
5#
Jim Fasarakis Hilliard Reply to 2017-10-27 18:54:12Z

Yes, this is a 'new style' object. It was a feature introduced in python2.2.

New style objects have a different object model to classic objects, and some things won't work properly with old style objects, for instance, super(), @property and descriptors. See this article for a good description of what a new style class is.

SO link for a description of the differences: What is the difference between old style and new style classes in Python?

Michael Currie
6#
Michael Currie Reply to 2014-04-28 19:24:35Z

Python 3.x:
class MyClass(object): = new-style class
class MyClass: = new-style class (implicitly inherits from object)

Python 2.x:
class MyClass(object): = new-style class
class MyClass: = OLD-STYLE CLASS

Explanation:

When defining base classes in Python 3.x, you’re allowed to drop the object from the definition. However, this can open the door for a seriously hard to track problem…

Python introduced new-style classes back in Python 2.2, and by now old-style classes are really quite old. Discussion of old-style classes is buried in the 2.x docs, and non-existent in the 3.x docs.

The problem is, the syntax for old-style classes in Python 2.x is the same as the alternative syntax for new-style classes in Python 3.x. Python 2.x is still very widely used (e.g. GAE, Web2Py), and any code (or coder) unwittingly bringing 3.x-style class definitions into 2.x code is going to end up with some seriously outdated base objects. And because old-style classes aren’t on anyone’s radar, they likely won’t know what hit them.

So just spell it out the long way and save some 2.x developer the tears.

Jim Fasarakis Hilliard
7#
Jim Fasarakis Hilliard Reply to 2017-12-17 08:59:43Z

History from Learn Python the Hard Way:

Python's original rendition of a class was broken in many serious ways. By the time this fault was recognized it was already too late, and they had to support it. In order to fix the problem, they needed some "new class" style so that the "old classes" would keep working but you can use the new more correct version.

They decided that they would use a word "object", lowercased, to be the "class" that you inherit from to make a class. It is confusing, but a class inherits from the class named "object" to make a class but it's not an object really its a class, but don't forget to inherit from object.

Also just to let you know what the difference between new-style classes and old-style classes is, it's that new-style classes always inherit from object class or from another class that inherited from object:

class NewStyle(object):
    pass

Another example is:

class AnotherExampleOfNewStyle(NewStyle):
    pass

While an old-style base class looks like this:

class OldStyle():
    pass

And an old-style child class looks like this:

class OldStyleSubclass(OldStyle):
    pass

You can see that an Old Style base class doesn't inherit from any other class, however, Old Style classes can, of course, inherit from one another. Inheriting from object guarantees that certain functionality is available in every Python class. New style classes were introduced in Python 2.2

kmario23
8#
kmario23 Reply to 2016-10-20 05:37:38Z

The syntax of the class creation statement:

class <ClassName>(superclass):
    #code follows

In the absence of any other superclasses that you specifically want to inherit from, the superclass should always be object, which is the root of all classes in Python.

object is technically the root of "new-style" classes in Python. But the new-style classes today are as good as being the only style of classes.

But, if you don't explicitly use the word object when creating classes, then as others mentioned, Python 3.x implicitly inherits from the object superclass. But I guess explicit is always better than implicit (hell)

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