Question
Why do Python classes inherit object?
Why does the following class declaration inherit from object
?
class MyClass(object):
...
Question
Why does the following class declaration inherit from object
?
class MyClass(object):
...
Solution
Is there any reason for a class declaration to inherit from
object
?
In Python 3, apart from compatibility between Python 2 and 3, no reason. In Python 2, many reasons.
In Python 2.x (from 2.2 onwards) there's two styles of classes depending on the presence or absence of object
as a base-class:
"classic" style classes: they don't have object
as a base class:
>>> class ClassicSpam: # no base class
... pass
>>> ClassicSpam.__bases__
()
"new" style classes: they have, directly or indirectly (e.g inherit from a built-in type), 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:
classmethod
: A method that receives the class as an implicit argument instead of the instance.staticmethod
: A method that does not receive the implicit argument self
as a first argument.property
: Create functions for managing the getting, setting and deleting of an attribute. __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.
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]
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 since Python inserts it for you behind the scenes.
Solution
Python 3
class MyClass(object):
= New-style classclass MyClass:
= New-style class (implicitly inherits from object
)Python 2
class MyClass(object):
= New-style classclass MyClass:
= OLD-STYLE CLASSExplanation:
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.
Solution
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?
Solution
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
Solution
Yes, it's historical. Without it, it creates an old-style class.
If you use type()
on an old-style object, you just get "instance". On a new-style object you get its class.
Solution
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)