Why do Python classes inherit object?

Why does the following class declaration inherit from object?

class MyClass(object):
    ...

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.

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 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__
(,)
>>> class IntSpam(int):              # indirectly inherit from object...
...    pass
>>> IntSpam.__bases__
(,) 
>>> IntSpam.__bases__[0].__bases__   # ... because int inherits from 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.

• properties with 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.

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 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 since Python inserts it for you behind the scenes.

Stack Overflow Q#4015417, score: 1265