We Keep Coding

Rewrite old-style classes as new ones

I ran pylint on some code and got a complaint about old-style classes.
Can I rectify this by simply changing:
class MyClass:
to:
class MyClass(object):
Or is there something more involved?

In Python 2, writing
class MyClass(object):
would suffice. Or you switch to Python 3, where
class MyClass:
would be just fine.
The inheritance list usually gives a list of base classes (see Customizing class creation for more advanced uses), so each item in the list should evaluate to a class object which allows subclassing. Classes without an inheritance list inherit, by default, from the base class object; hence
class Foo:
pass
is equivalent to
class Foo(object):
pass
See also: https://docs.python.org/3/reference/compound_stmts.html#class
Also, as #Kevin pointed out in a comment, method resolution is not trivial and might lead to unexpected behavior when using multiple inheritance: http://python-history.blogspot.com/2010/06/method-resolution-order.html

Should all Python classes extend object? [duplicate]

This question already has answers here:
Why do Python classes inherit object?
(6 answers)
Closed 1 year ago.
I have found that both of the following work:
class Foo():
def a(self):
print "hello"
class Foo(object):
def a(self):
print "hello"
Should all Python classes extend object? Are there any potential problems with not extending object?

In Python 2, not inheriting from object will create an old-style class, which, amongst other effects, causes type to give different results:
>>> class Foo: pass
...
>>> type(Foo())
<type 'instance'>
vs.
>>> class Bar(object): pass
...
>>> type(Bar())
<class '__main__.Bar'>
Also the rules for multiple inheritance are different in ways that I won't even try to summarize here. All good documentation that I've seen about MI describes new-style classes.
Finally, old-style classes have disappeared in Python 3, and inheritance from object has become implicit. So, always prefer new style classes unless you need backward compat with old software.

In Python 3, classes extend object implicitly, whether you say so yourself or not.
In Python 2, there's old-style and new-style classes. To signal a class is new-style, you have to inherit explicitly from object. If not, the old-style implementation is used.
You generally want a new-style class. Inherit from object explicitly. Note that this also applies to Python 3 code that aims to be compatible with Python 2.

In python 3 you can create a class in three different ways & internally they are all equal (see examples). It doesn't matter how you create a class, all classes in python 3 inherits from special class called object. The class object is fundamental class in python and provides lot of functionality like double-underscore methods, descriptors, super() method, property() method etc.
Example 1.
class MyClass:
pass
Example 2.
class MyClass():
pass
Example 3.
class MyClass(object):
pass

Yes, all Python classes should extend (or rather subclass, this is Python here) object. While normally no serious problems will occur, in some cases (as with multiple inheritance trees) this will be important. This also ensures better compatibility with Python 3.

As other answers have covered, Python 3 inheritance from object is implicit. But they do not state what you should do and what is convention.
The Python 3 documentation examples all use the following style which is convention, so I suggest you follow this for any future code in Python 3.
class Foo:
pass
Source: https://docs.python.org/3/tutorial/classes.html#class-objects
Example quote:
Class objects support two kinds of operations: attribute references
and instantiation.
Attribute references use the standard syntax used for all attribute
references in Python: obj.name. Valid attribute names are all the
names that were in the class’s namespace when the class object was
created. So, if the class definition looked like this:
class MyClass:
"""A simple example class"""
i = 12345
def f(self):
return 'hello world'
Another quote:
Generally speaking, instance variables are for data unique to each
instance and class variables are for attributes and methods shared by
all instances of the class:
class Dog:
kind = 'canine' # class variable shared by all instances
def __init__(self, name):
self.name = name # instance variable unique to each instance

in python3 there isn't a differance, but in python2 not extending object gives you an old-style classes; you'd like to use a new-style class over an old-style class.

Python: Reference to a class generically in an instance method?

This question is similar, but it pertains to static methods: In Python, how do I reference a class generically in a static way, like PHP's "self" keyword?
How do you refer to a class generically in an instance method?
e.g.
#!/usr/bin/python
class a:
b = 'c'
def __init__(self):
print(a.b) # <--- not generic because you explicitly refer to 'a'
#classmethod
def instance_method(cls):
print(cls.b) # <--- generic, but not an instance method

For old-style classes (if your code is Python 2.x code, and your class in not inheriting from object), use the __class__ property.
def __init__(self):
print(self.__class__.b) # Python 2.x and old-style class
For new-style classes (if your code is Python 3 code), use type:
def __init__(self):
print(self.__class__.b) # __class__ works for a new-style class, too
print(type(self).b)
Internally, type uses the __class__ property.

Python Default Inheritance?

If I define a class in Python such as:
class AClass:
__slots__ = ['a', 'b', 'c']
Which class does it inherit from? It doesn't seem to inherit from object.

If you define a class and don't declare any specific parent, the class becomes a "classic class", which behaves a bit differently than "new-style classes" inherited from object. See here for more details: http://docs.python.org/release/2.5.2/ref/node33.html
Classic classes don't have a common root, so essentially, your AClass doesn't inherit from any class.
Note that this is specific to Python versions before 3.0. In Python 3, all classes are new-style classes and inherit implicitly from object, if no other parent is declared.

Try the following code snippet in Python 2.7 and Python 3.1
class AClass:
__slots__ = ['a', 'b', 'c']
print(type(AClass))
print(issubclass(AClass,object))
print(isinstance(AClass,type))
In Python 2.7, you will get:
<type 'classobj'>
False
False
And Python 3.1 you will get.
<class type>
True
True
And that explains it all. It is old style class in Python 2, unless you subclass it from object. Only in Python3, it will be treated like a new style class by default.

Let's try it out.
>>> class AClass:
... pass
...
>>> AClass.__bases__, type(AClass)
( (), <type 'classobj'> ) # inherits from nothing
>>> class AClass(object): # new style inherits from object
... pass
...
>>> AClass.__bases__, type(AClass)
( (<type 'object'>,), <type 'type'> )
Read an introduction to the new style classes in the links given in other answers.

In Python 2.x or older, your example AClass is an "old-style" class.
A "new-style" class has a defined inheritance and must inherit from object or some other class.
What is the difference between old style and new style classes in Python?
EDIT: Wow, I didn't think it was possible to use the old-style syntax in Python 3.x. But I just tried it and that syntax still works. But you get a new-style class.

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__
(<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.
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.

Python 3
class MyClass(object): = New-style class
class MyClass: = New-style class (implicitly inherits from object)
Python 2
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.

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?

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

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.

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)
Reference